Spectra Precision Survey Pro CE v3.60 User Manual
SURVEY PRO
for Windows® CE
GPS User’s Manual
2002 Tripod Data Systems, Inc.
All Rights Reserved
GPS User’s Manual
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.MAN-CESURVPROGPS 10152002
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ii
Table of Contents
Introduction........................................................................................... 1
GPS Coordinates................................................................................... 2
Datums .......................................................................................2
Horizontal Datums....................................................................................4
Vertical Datums.........................................................................................6
Coordinate Systems..................................................................8
Horizontal Coordinate Systems.............................................................10
Vertical Coordinate Systems..................................................................14
Survey Pro Coordinate System Database.............................................16
GPS Measurements............................................................................17
Pseudo-Range Positioning.....................................................17
Differential GPS.......................................................................................17
Differential GPS with Survey Pro.........................................19
RTK Data Collection..........................................................................21
Setting Projection Mode.........................................................21
Projection Mode Summary.....................................................................22
Mapping Plane Setup..............................................................................24
Selecting Geoid Model ............................................................................27
Using Ellipsoid Heights ..........................................................................28
Receiver Setup.........................................................................28
General Hardware Configuration.........................................................28
Base Station Receiver Setup ...................................................................30
Rover Receiver Setup..............................................................................33
Solve Localization...................................................................34
Localization with Control Points...........................................................35
Localization Parameters Explained.......................................................41
One Point Localizations..........................................................................46
RTK Data Collection...............................................................49
Measure Mode .........................................................................................49
Data Collection Methods........................................................................51
RTK Stake Out.........................................................................55
Bluetooth Communication.....................................................56
Configuring the Bluetooth Settings.......................................................56
Bluetooth Limitations..............................................................................58
Recovering from Signal Loss..................................................................59
iii
GPS User’s Manual
Bluetooth Error Messages ......................................................................59
Projection Utilities..............................................................................61
Projection Calculator ..............................................................61
Scale Factor Calculator ...........................................................................61
Convergence Calculator .........................................................................62
Readjust Points........................................................................62
Geodetic to Plane.....................................................................................62
Plane to Geodetic .....................................................................................63
Managing GPS Coordinates with TDS...........................................66
Survey Pro................................................................................66
Manual Mode...........................................................................................66
Edit Points ................................................................................................67
Import a .GPS File ...................................................................................68
Survey Link..............................................................................69
File Import................................................................................................69
File Export ................................................................................................70
Post Processing Data Collection.......................................................71
Field Procedure .......................................................................71
Turn On Post Processing........................................................................71
Start Recording in Receiver....................................................................72
Data Collection ........................................................................................72
Office Procedure......................................................................74
Tutorial Jobs ........................................................................................75
Trouble Shooting................................................................................87
References............................................................................................91
iv
Introduction
This book is divided into two parts. The first part is the user’s
manual. The second part is the reference manual.
The users manual includes a brief explanation of the basic concepts of
GPS coordinate systems and GPS measurements. The following
sections cover step-by-step instructions on how to use Survey Pro
GPS for RTK and post processing data collection. At the end of the
user's manual are some tutorial jobs you can do to illustrate the
instructions in the book.
The reference manual contains descriptions and illustrations of every
Survey Pro GPS screen. The reference ma nual is divided into
sections based on the S u rvey Pro GPS menus. To find the description
of a particular screen, simply refer to the section dealing with that
menu and screen.
GPS Coordinates
To represent positions in space you need two things. First, you need
a datum to define an origin, an orientation, and a scale. Second, you
need a coordinate system to specify the locations in the datum. GPS
positions are in a global geocentric datum, us ing latitude and
longitude angles to specify location. Most engineering and surveying
jobs require positions in a 2D Cartesian coordinate system. In order
to use GPS with most coordinate systems, we must transform the
GPS measurements into local coordinates.
Some jobs require a coordinate system based on a specified mapping
plane and geodetic datum, such as a UTM. For this case, the section
below describes how Survey Pro converts WGS84 GPS positions into
local north and east coordinates. Other jobs have an arbitrary
coordinate system, such as a re-survey of an old conventional traverse
or starting a new job from scratch. For this case, you can use TDS
Localization, and you will not need to know the details of anything in
this section.
Datums
A datum consists of three components: an origin, an orientation, and
a scale. The origin defines the start point, the orientation defines the
direction of the bearings, and the scale defines the relative magnitude
of the distance units. For example, a surveyor shows up at a new job
site, places a monument in the ground and calls it (5000, 5000, 100).
This establishes the origin of the datum. The surveyor does a sun
shot and calculates the azimuth to a reference object. Astronomic
north at this meridian establishes the orientati on of the datum.
Finally, the surveyor begins measuring distances with a total station.
The EDM establishes the scale of th e datum.
GPS measurements are taken in a global geocentric da tum, the World
Geodetic System of 1984 (WGS84). The WGS84 datum has its origin
at the earth’s center of mass, its orientation defined by the earth’s
spin axis and the int ersection of the mean meridian of Greenwich
with the mean equatorial plane, and its scale defined by metric
standard measurement.
2
GPS Coordinates
Geocentric datums such as WGS84 use a rotational ellipsoid to model
the shape of the earth. The WGS84 ellipsoid was based on and is
virtually identical to the Geodetic Reference System of 1980 (GRS80)
ellipsoid. The ellipsoid origin is at the earth’s center of mass. Its
minor axis corresponds with the earth’s rotation axis and its major
axis corresponds to the mean equatorial plane.
WGS84 Geodetic v. Local Geodetic
When the coordinate system i s a mapping plane in a datum ot her
than WGS84, positions measured in WGS84 latitude, longitude and
height, must be transformed into local latitude, longitude, and height
before they can be used to calculate nort hing and easting with the
specified map projection.
There are three methods of datum transformation supported by
Survey Pro.
• Molodensky Transformation: Is the most commonly used
transformation. Three parameters specify an X,Y,Z s h ift between
WGS84 and the local datum origin. Survey Pro uses the
Molodensky datum transformation algorithms specified in the
National Imagery and Mapping Agency T echnical Report
8350.2.1.
• Similarity Transformation: The most precise method of datum
transformation. The seven-parameter similarity transformation,
also called the Helmert transformation, uses a shift of XYZ origin,
a rotation about XYZ axes, and a scale to transform from WGS84
and the local datum.
• Grid File Datum Transformation: Is used when the datum
differences are not consistent over large areas. A grid file datum
transform uses a data set of shift values. For any location, an
approximate shift can be calculated by interpolating from the
data set.
For many surveying applications, the horizontal and vertical datums
are separate. This is because GPS heights are measured on the
ellipsoid with its origin at the earth's center of mass, while elevation
is a function of local gravity field, which is influenced by the unequal
distribution of mass in the earth.
1
http://164.214.2.59/GandG/tr8350_2.html
3
GPS User’s Manual
Below is a description of some common horizontal and vertical
datums used by Survey Pro.
Horizontal Datums
•
NAD27
The North American Datum of 1927 (NAD27) horizontal datum was
established in t h e early part of the twentieth century to define a
horizontal coordinate system in North America. The datum
originated at a central point, Meades Ranch in Kansas. From there,
conventional triangulation and trilateration networks radiated
outward to establish new monuments in the system. The datum was
based on the Clarke 1866 ellipsoid, which was the best fitting
ellipsoid for the North American continent at the time.
Survey Pro performs a grid transformation for NAD27 in the United
States using the NADCON datum sets in *.DGF (datum grid file)
format. Several specific Molodensky datum transformations are also
available for other areas in North America.
Note: To use a grid datum, you must have the pair of *.dgf files for
latitude and longitude shift the Disk\Geodata directory.
•
NAD83 = WGS84
Later in the twentieth century, satellite and Very Long Baseline
Interferometry (VLBI) measurements were a dded to the numerous
conventional measurement networks and re-adjusted to define the
North American Datum of 1983 (NAD83). NAD83 was created to
conform to the new global datum, WGS84, and uses the same
reference ellipsoid.
Survey Pro uses no datum transformation for NAD83. Therefore,
NAD83 = WGS84 in Survey Pro projection calculations.
•
NAD83 ≠ ITRF WGS84(1996.0 , 1997.0 , …)
Continuing improvements in GPS and VLBI technology as well as
increased cooperation among world wide agencies, like the
International Earth Rotation Society (IERS), led to a much better
solution for the Earth’s center of mass and spin axis. The IERS’s
solution is adopted as the International Terrestrial Reference Frame
4
GPS Coordinates
(ITRF). Because the earth’s center of mass and spin axis drift over
time, you will often see the WGS84 datum followed by brackets
(1996.0). The date in the brackets indicates the epoch defining the
datum.
This is all quite confusing. Fortunately, for most RTK GPS
applications, you do not need to worry about these WGS84
differences. The significant part of the datum difference is a shift,
and you correct this when you specify the GPS base position. The
other part of the datum difference is the small rotation of the axes.
These rotations are small enough to ignore except for the most precise
first order applications.
If your Survey Pro job requires a local datum in one epoch of WGS84
and the WGS84 datum in a different epoch, you can setup a sevenparameter similarity transformation. For the transformation
parameters of any epoch of WGS84 an d for a more detailed
description of the simila rity transformation and WGS84, see NGS
web site
•
2
.
High Accuracy Reference Network (HARN)
In the United States, the bulk of the measurements used to establish
NAD83 were conventional. These measurements contain sli ght
systematic errors that conflict with GPS measurements, which are
more precise over long distances. To address this problem in the
U.S.A., in 1988 the National Geodetic Survey (NGS) began to update
NAD83 coordinate datums with HARN GPS surveys on a state-bystate basis. These HARN surveys determined small (< 5_cm)
corrections to the location of A and B order control monuments across
the states.
Survey Pro performs a grid transformation for HARN networks in the
United States using the NADCON datum files in *.DGF format.
Note: To use a grid datum, you must have the pair of *.dgf files for
latitude and longitude shift the Disk\Geodata directory.
2
Snay, R. How CORS Positions and Velocities Were Derived.
http://www.ngs.noaa.gov/CORS/Derivation.html
Appendix B.
5
GPS User’s Manual
•
Custom Datum Transformations
Most North American and international datums are pre programmed
into the Survey Pro coordinate system database. If you require a
datum not programmed into the database, you can use the Projection
Key In Wizard to create a custom Molodensky or similarity datum
transformation.
Vertical Datums
GPS satellites orbit the Earth’s center of mass, while objects on the
surface of the planet are affected by the force of the local gravity field.
Although it is possible to accurately model the orbits of satellites
about the Earth’s center of mass, modeling the local gravity field is
much more difficult because of the unequal distribution of masses
within the earth.
We all know that water flows downhill from a higher elevation to a
lower one. However, water will not always flow from a higher
ellipsoid height to a lower one. Ellipsoid height is simply the altitude
above the reference surface, and may not match the slope of the local
gravity field. When surveying with GPS, we need to correct for the
local gravity field to convert measured ellipsoid heights (h) into
orthometric elevations (H). This is usually done with a geoid model.
Survey Pro can use several different geoid models to convert local
ellipsoid heights into elevations in a particular vertical datum. Most
geoid models are initially based on the global equipotential surfac e
used in the definition of the initial WGS84 datum. Below is a
description of some vertical geoid models and datums.
Vertical Datums
•
EGM96
The National Imagery and Mapping Agency publishes the globa l
geopotential model EGM96
generate the worldwide 15-minute geoid height grid data file,
WW15mGH.grd. This file contains geoid separation values at 15minute intervals for the entire globe and provides a good esti mate of
geoid slope corrections.
3
http://164.214.2.59/GandG/wgs-84/egm96.html
6
3
. This geopotential model was used to
GPS Coordinates
•
NGVD29
The first continental height datum in the United States was the
National Geodetic Vertical Datum of 1929 (NGVD29). According to
the technology of the day, this datum was based on normal gravity,
that is, the gravity field at the instrument when it was leveled.
Points along the coast were chosen and their elevation above sea level
was determined from a network of tide gauges. Spirit level networks
were then run across the country and closed on the opposite coast.
This datum contained a number of systematic errors including unmodeled local gravity effects and refraction errors. Also, it was later
discovered that the “mean sea level” from the Atlantic to the Pacific
Oceans was not the same.
•
NAVD88
In an effort to address these errors, the North American Vertical
Datum of 1988 (NAVD88) was realized from a single datum point in
Rimouski, Quebec. This datum is based on actual gravity, which
provides a better representation of true orthometric elevations. The
primary consideration in the choice of this datum point was to
minimize the recompilation of national mapping products. A side
effect of this choice is that the NAVD88 datum and the theoretical
level surface used to define GRS80 do not agree. The offset b etween
the NAVD88 vertical datum and the ITRF global geopotential model
is in the neighborhood of 0.27m
4
.
Survey Pro does not require choice of a specific vertical datum. For
RTK applications, elevations are solved relative to the base using the
vertical localization adjustment. Therefore, the vertical datum is
established by the datum of the base elevation.
4
Milbert D.G. Converting GP S Height into NAVD88 Elevation with
the GEOID96 Geoid Height Model
http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html
p. 4
7
GPS User’s Manual
Coordinate Systems
A coordinate system is a way to describe positions in a datum.
Coordinate systems range from simple Cartesian (y,x) or (N,E)
positions on a flat plane to complex geodetic lati tudes and longitudes
on a reference ellipsoid.
Below is a description of some coordinate systems common in
surveying:
•
Northing, Easting, Elevation
Survey projects usually use simple plane coordinates. We assume our
local datum models a flat earth, and we calculate coordinat es in a
Cartesian syst em where the simple laws of plane t rigonometry apply.
When a vertical coordinate is required, most survey projects require
orthometric elevations.
•
Lat, Lng, Ht
Geodetic horizontal coordinates are usually expressed as two angles
called latitude and longitude (φ, λ). Geodetic vertical coordinates are
usually expressed as the distance above the ellipsoid called height.
The angles describe a point’s position on the surface of the reference
ellipsoid. The height describes the altitude normal to the surface of
the reference ellipsoid.
X
c
h
M
e
r
i
d
i
a
n
Z
G
r
e
e
n
w
i
h
φ
λ
r
o
t
a
u
q
E
Y
8
Fig. 1: Ellipsoidal Geodetic
Coordinates
GPS Coordinates
•
ECEF XYZ
Geodetic coordinates are some times given in the Earth Centered
Earth Fixed (ECEF) Cartesian coordinate system. This coordinate
system has its origin at the Earth’s center of mass, the primary (Z)
axis is the earth’s spin axis; the secondary (X) axis is the intersection
of the equatorial plane an d the mean meridian of Greenwich; the
tertiary (Y) axis is orthogonal in a right handed system. An ECEF
XYZ coordinate can be converted into the corresponding lat, lng, ht
using standard formulas.
i
c
h
M
e
r
i
d
i
a
n
X
Z
y
G
r
e
e
n
w
λ
a
u
q
E
x
h
z
φ
r
o
t
Y
Fig. 2: Earth Centered Earth
Fixed Geodetic Coordinates
For many surveying applications, the horizontal and vertical
coordinate systems are separate. Below are descriptions of common
horizontal and vertical coordinate systems used in surveying and
mapping.
9
GPS User’s Manual
Horizontal Coordinate Systems
Survey projects use horizontal coordi nates on either a local plane or a
map projection. For small projects, you can assume a simple flat
earth plane and calculate coordinates directly with measured
distances. Use TDS localization mode for this procedure. For large
projects, a mapping plane is used to accurately represent the curved
surface of the earth on a flat plane and conventionally measured
distances need to be scaled to the mapping plane grid.
Map Projections
A map projection uses equations to transform local lat itude and
longitude into (y,x) Cartesian coordinates on a flat plane. Map
projections attempt to minimize distortions to the following
properties
•
A map projection is conformal when local angles are preserved.
Conformal maps are important for surveying because, for second
order surveys, angles measured on the ground are angles on the map.
Meridians (lines of longitude) and parallels (lines of latitude)
intersect at right angles and shape is preserved locally. The physical
characteristic of conformality is that the scale factor at any point on
the map is the same in all directions.
5
:
Conformality
•
Distance
A map projection is equidistant when it correctly plots distances from
the center of the projection to any other place on the map. Most map
projections involve some distortion of scale. Consequently, when
converting distances measured on the ground to distances on the grid,
a scale factor must be applied.
•
Direction
A map projection is azimuthal when azimuths (angles from a point on
a line to another point) are correctly plotted in all directions.
5
http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
10
GPS Coordinates
•
Area
A map projection is equi-areal when it correctly plots areas over the
entire map. That is, all mapped areas have the same proportional
relationship to the areas on the Earth that they represent.
Common Conformal Map Projections in Surveying
•
Transverse Mercator
The Transverse Mercator (TM) projection results from projecting the
ellipsoid onto a cylinder tangent to a central meridian. Scale
distortion is maximum from east to west and minimum from north to
south, so the TM projection is often used to portray areas with large
north-south extent. Distortion of scale, distance, direction and area
increase away from the central meridian.
Many national grid systems are based on the TM projection. The
Universal Transverse Mercator grid system divides the world into 60
6-degree zones. About half of the U.S. states use a TM projection for
their State Plane Coordinate Systems. The British National Grid
(BNG) is a TM projection with origin at 49 degrees north lat itude and
2 degrees west longitude.
•
Oblique Mercator
The Oblique Mercator projection is similar to the Transverse
Mercator projection; the ellipsoid is projected onto a cylinder.
However, instead of the cylinder tangent to the ellipsoid along a
meridian, it is tangent to the ellipsoid along any great circle other
than the Equator or a meridian. This makes the Oblique Mercator
projection appropriate for regions centered along lines, which are
neither meridians nor parallels.
The Oblique Mercator projection is used for Alaska State Plane zone
1, which covers the panhandle.
•
Lambert Conformal Conic
The Lambert Conformal Conic projecti on results from projecting a
sphere onto a cone tangent at two (or one) parallels of longitude.
Scale distortion is maximum from north to south and minimum from
east to west, so the Lambert projection is used to map areas of large
east-west extent. Distortion of scale, distance, direction and area
increase as you move away from the standard parallels.
11
GPS User’s Manual
Lambert projections are used for about half of the State Plane
Coordinate System zones in the USA.
•
Stereographic
The Stereographic projection results from projecting an ellipsoid onto
a plane. Directions are true from the center point and distortions in
scale, area and shape increase uniformly away from the central point.
The stereographic projection is azimuthal.
Because the scale is distorted somewhat uniformly in all directions,
stereographic map projections are a good representation of a
surveyor’s typical flat earth ground coordinate system. For this
reason, the stereographic ma p projection is used by the TDS
localization algorithm to convert (lat,lng) into local ground level
coordinates. For more information on localization, see page 35.
Scale Factors
When converting distances on a map to distances on the ground, you
must correct for two different scale distorti ons. First, the effects of
the map projection distortion must be corrected with the mapping
plane scale factor. Second, the geometric effect of your height above
the reference surface (ellipsoid height) must be corrected with the
ellipsoid scale factor. Generally, these two scale factors are
multiplied together into the combined scale factor.
Mapping Plane Scale Factor
This scale factor accounts for the distortions caused by the mapping
plane equations as they fit a curved surface onto a flat plane. It is a
function of the mapping plane equations and its exact value depends
on your location on the map. Although the scale factor is computed
with differential equations of the map projection, one can vis u alize it
in a geometric sense. Consider the following diagrams:
12
K=1.0 K=1.0
K>1.0
Mapping Plane
d
i
o
s
p
i
l
l
E
Fig. 3: Transverse Mercator Mapping Plane
K<1.0
K=0.9996
Central Meridian
K>1.0
Distance on Ellipsoid
Distance on Map
A side view of the cylinder shows the effect of scale
distortion.
•
Mapping
Plane
TDS Localization Stereographic Projection
The scale factor at the start location (usua lly the first
GPS base station in a project) is calculated for
h
Terrain
ground distances at the base height. The scale factor
increases more or less uniformly in all directions as
you move away from the base. The scale factor does
not change appreciable within the range of RTK
Ellipsoid
GPS, so this map projection is an excellent way to
model simple flat earth ground level
Fig. 4: Localization Stereographic Mapping
Plane
coordinate systems.
A side view of the ellipsoid and
stereographic mapping plane show the
scale calculated for ground distances at
the base height.
GPS Coordinates
•
Universal Transverse Mercator
Projection
The scale factor at th e central
meridian (CM) is 0.9996. The
scale factor is 1.0 approximately
170-km east and west of the CM.
The scale factor is less than one
between the CM and the point of
tangency. The scale factor is
greater than one beyond the point
of tangency. Therefore, at the
central meridian, a geodetic
distance of 100m scales into a
mapping plane distance of
99.96m.
13
GPS User’s Manual
φ
Ellipsoid Scale Factor
This scale factor accounts for the height of the ground
above the reference surface (the ellipsoid). This scale
factor is defined geometrically: Consider the following
diagram:
dist
dist
k
elip
R
/(R+h) = dist
ground
/dist
elip
ground
= R/ (R+h)
=
a
e
−
/R
elip
= R/(R+h)
1
22
2
)sin1(
Ellipsoid
Height, h
Radius of
Ellipsoid, R
Fig. 5: Ellipsoid Scale Factor
Combined Scale Factor
Generally, the two scale factors are multiplied together into a
combined scale factor. The combined scale factor is then a pplied to
grid distances to get ground distances:
k
cf
dist
= k
grid
* k
elip
= dist
map
ground
* k
cf
Distance on
Ground
Distance on
Ellipsoid
R+h
Vertical Coordinate Systems
GPS measurements provide ellipsoid heights. Most
survey projects require orthometric elevations. To
convert heights into elevations, you need to correct
for the difference between the surface of the
reference ellipsoid and the level surface representing
the gravity field.
The procedure to convert heights (h) to elevations (H)
involves the use of a geoid model. The geoid is a
theoretical surface that approximates mean sea
level. If one knows the separation between the reference ellipsoid
and the geoid, called the geoid undulation (N), then one can
determine orthometric elevations from ellipsoidal heights.
14
Terrain
Geoid
Ellipsoid
H
h
N
h=H+N
Fig. 6: The Height Equation
GPS Coordinates
Geoid Models in TDS Software
Survey Pro has several geoid models in the coordinate syst em
database. All of the geoid models use data files in geoid grid file
(*.GGF) format.
Note: To use a geoid model, geoid data files (*.GGF) must be in t he
Geodata directory.
• Users in the U.S.A., Mexico, and the Caribbean can use either the
NGS Geoid96 or the NGS Geoid99 models. This coverage
includes the continental United States, Alaska, Hawaii, Mexico,
and the Caribbean.
• Users in Canada can use the Geodetic Survey Division CGG2000
model. See the GSD web site for available data sets.
• Users anywhere in the world can use the NIMA worldwide 15-
minute geoid height grid data file, WW15mGH.ggf. This file
covers the entire globe on a 15’ x 15’ grid.
• Users in Australia can use the AUSGEOID98 data set. This data
set is available in several overlapping sub grid files in the *.GGF
format at the TDS web site.
Note: To use any of these geoids with Survey Pro, you need to
convert the files from their native format into the *.GGF format. This
is done with the Geoid Sub Grid function in survey Link. T h e
required NGS and the NIMA geoid model data files are supplied free
of charge on the TDS Survey Works CD. The Canadian GSD geoid
models are a licensed data set, so you have to contact Natural
Resources Canada to obtain the data in the NGS.bin format to use
with the Survey Link Geoid Sub Grid extraction utility. The
Australian geoid model is available in *.GGF format for download at
the TDS web site on the GPS page.6
6
Go to www.tdsway.com and select Downloads.
15
GPS User’s Manual
Survey Pro Coordinate System
Database
Survey Pro uses a Coordinate System Database file (*.CSD) to store
the map projection and datum transformation parameters for many
different coordinate systems around the world. Also, horizontal and
vertical localization adjustments are stored as site records in the
database. Below is a list of the terminology used to describe the
different records in the coordinate system database.
• Zone: Is the basic record type. A zone record specifies the type of
map projection and its parameters. Most zone records have
datum and ellipsoid records, and usually a geoid record, already
in the database.
• Site: Is a zone record with a horizontal and/or vertical
localization adjustment added. Localizations are usually used to
correct GPS positions starting from an autonomous base. They
can be computed from control points or from manual input of
parameters.
• Zone Group: Is a collection of zone and/or site records used to
keep the database organized and user selection easier.
• Datum: Is a type of datum transformation and its parameters. A
datum record will always have an ellipsoid record already in the
database.
• Ellipsoid: Are the two parameters specifying the ellipsoid of the
datum.
• Geoid: Is a geoid model and its associated data file.
16
GPS Measurements
This section gives a brief explanation of GPS measurements. First, a
discussion of the basic theory of differential positioning will
familiarize you with different solution types and their expected
precision. Next, step-by-step instructions will describe how to
configure Survey Pro with GPS receivers to perform either Real Time
Kinematic (RTK) GPS or data collection for post processing
differential GPS.
Pseudo-Range Positioning
GPS solutions are computed using pseudo-range positioning: Position
is determined from multiple pseudo-range measurements to different
satellites (or space vehicles SV) at a single measurement epoch. The
position of the GPS receiver antenna is computed by intersecting the
pseudo-ranges from the known SV position in a manner similar to
survey resection. Four SVs are required to determine three position
dimensions and time. Posit ion dimensions are computed by the
receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ)
coordinates.
A pseudo-range solution will be one of two types: autonomous, or
differential. A single GPS receiver can compute an autonomous
position from ranges to four or more SV. This single receiver position
is extremely coarse. One can expect errors in the order of 100-m on a
bad day. For this reason, any precise GPS must be performed using
differential positioning.
Differential GPS
Differential GPS (DGPS) positioning involves subtracting a
combination of ranges measured to various satellites from multiple
receivers. When the signals are subtracted, the major error sources
cancel each other out. However, becaus e you are computing a
difference in ranges, the DGPS measurement solves for a coordinate
difference and not a coordinate. To compute a coordinate using a
coordinate difference, you must specify a starting point.
17
GPS User’s Manual
Differential Solutions: Types and Quality
Code Differential
Code differential solutions use the Coarse Acquisition (C/A)
navigation code transmitted on the GPS carrier wave. Because the
wavelength of the code segment is long (300m), code differential is the
least precise differential solution. Accuracies of 1-10 meters are
possible with DGPS using C/A code differential positioning.
Carrier Phase Differential
Highly precise coordinate differences can be measured using pseudorange positioning with the carrier signal wave. Because t h e
wavelength of the carrier wave is only 19 cm, mm level positioning is
possible. When the signal arrives at the antenna, we can measure
the fractional part of the ca rrier wave. If we can then calculate the
whole number of wavelengths between the SV and the antenna (the
ambiguity), we can add it to the fractional part and multiply by the
length of one cycle to measure a precise range.
Calculating the exact number of wavelengths uses a complicated least
squares process, which is often called ambiguity resolution. The
ambiguity resolution will yield either a float or a fixed solution.
•
Fixed Solution
We know the number of wavelengths will be a whole number.
Techniques are used to constrain the least squares solution to yield a
whole number. If we get an acceptable solution, we say that this
solution is fixed. A fixed solution will generate coordinate differences
precise to about 15-ppm (si n gle frequency) or 5-ppm (dual frequency),
which translates into 15-mm or 5-mm over a 1-km base line.
Several things may prevent you from achieving a fixed solution: bad
multi-path, low number of satellites and bad constellation geometry,
poor radio link for corrections (RTK).
•
Float Solution
If the constraint algorithm does not produce an acceptable fixed
solution, then the ambiguity is allowed to be a decimal (float)
number. A float solution will generate coordinate differences precise
to about 100 to 500-ppm, which translates into 0.1-m to 0.5-m over a
1-km base line.
18
GPS Measurements
Differential GPS with Survey
Pro
DGPS requires raw data measured from separate receivers to be
combined into a single range difference. For Real Time Kinematic
(RTK) data collection, the raw data can be broadcast using a radio
link or cell phones and the differential solution is solved in real time.
For Post Processing data collection, the raw data is collected in each
receiver’s internal memory and downloaded to a PC. Then, software
is used to combine the raw data and produce the differential solution.
Survey Pro supports three different modes of differential GPS data
collection: RTK, Post Processing, and Simultaneous RTK and Post
Processing data collection.
Selecting Data Collection Mode
1. Go to Settings from the Job menu.
2. On the Receiver
drop down box.
Mode
3. Tap 2..
Note: This switch controls the display of the Survey
GPS mode. In RTK mode, the Survey menu contains items to
configure a base and rover receiver for RTK data collection. When in
RTK mode with post processing turned on, you will get simultaneous
post processing data collection every time you occupy a point. In Post
Process mode, the survey menu contains items to configure a st atic or
a stop and go receiver for post processing data collection.
card, select
RTK
or
Post Process
in the
menu when in
GPS
19
GPS User’s Manual
RTK Settings
If you are using Survey Pro for RTK, or RTK and post processing
simultaneous data collection, the following cards of the Job, Settings
screen contain settings specific to RTK:
• Measure Mode
occupations and the type of GPS raw data to store for each point.
You can also specify measurement acceptance criteria. For more
information see the Reference Manual.
• Projection
projection type and specify the path to geodetic data files. For
more information see the Reference Manual.
: is where you select the receiver dynamic for point
: is where you set your horizontal and vertical
Post Processing Settings
If you are using Survey Pro for post processing data collection, the
following settings apply.
• Post Process
by specifying a recording interval for the receiver’s internal
memory. You can also specify how to deal with a utonomous
positions and select a special layer to store autonomous points on.
For more information see the Reference Manual.
: is where you turn on post processing data collection
20
RTK Data Collection
RTK data collection uses differential GPS corrections broadcast by a
base receiver to solve for coordinates at a rover receiver in real time.
This section describes how to use Survey Pro for RTK GPS data
collection. Topics include:
How to select a projection method
¾
How to configure the base and rover hardware
¾
How to set a base point in the Survey Pro software
¾
How to collect control points and solve the horizontal and
¾
vertical projections
How to collect data and stake out measurements
¾
Setting Projection Mode
When you open a new job in Survey Pro, the
Ground - TDS Localization
ground coordinates in any arbitrary coordinate system, such as a
resurvey of an old job or a brand new job from a single start point.
Using
datum or map projection. Survey Pro will automatically initialize a
default projection for ground level dist ances when you configure the
first RTK base station in the job.
If your job requires a specified map projection and datum from the
coordinate system database, then you should switch
to
projection zone from the database or creates a custom zone and
datum transformation using Survey Pro.
In either horizontal projection mode, the user can choose one of two
methods for vertical projection:
Heights
vertical coordinate to be orthometric elevations.
to use the GPS measured height as the local vertical coordi nate.
Ground - TDS Localization
Mapping Plane
. Use
. In
Localization (+Geoid)
. This is the default mode to produce
, the user does not need to select a
Mapping Plane
mode the user selects a map
Localization (+Geoid)
mode when you want the local
Projection Mode
Projection Mode
, or
Use Ellipsoid Heights
is set to
Ellipsoid
21
GPS User’s Manual
Projection Mode Summary
Horizontal
Ground - TDS Localization
• Local coordinates are at ground level, based on the project height.
• Distances shot with EDM are at ground scale, so are 1:1 with
coordinates solved by the projection.
• Default map projection and datum are automatically initialized
with RTK base setup.
Mapping Plane
• Local coordinates are on a conformal map projection grid.
• Distances shot wi th EDM are usually scaled by the combined
scale factor to distances on the map projection grid.
• User selects map projection zone.
Vertical
Localization (+Geoid)
• Vertical coordinate is orthometric elevation.
• User must solve transformation from ellipsoid heights to
elevations. This is done with Localization on control points, or
using a geoid model, or a combination of both.
Ellipsoid Heights
• Vertical coordinate is ellipsoid height.
• This mode requires no transformation setup. Use this mode when
vertical coordinates do not need to be elevations.
If you are using
projection mode, and you are not using a geoid model, Survey Pro is
ready to start the RTK survey upon opening the new job. No
projection setup is necessary, so you should move ahead to the next
section on Configuring Receivers.
22
Ground- TDS Localization
for your horizontal
RTK Data Collection
Note: If you are using
projection mode, and you want to use a geoid, you only need to select
the geoid model once. Survey Pro remembers the geoid model you
last used and will automatically assign this geoid in a new job's
Localization map projection zone. You can go directly to Receiver
Setup after opening a job.
If you are using a map projection zone and/ or you want to use a geoid,
but have never selected one, you will need to choose a zone and/or
geoid record from the user interface. The following section describes
how to select a map projection and geoid model from the coordinate
system database.
1. Go to Projection
at the top of the screen to open the Projection card of the Settings
screen. Select the appropriate Horizontal and Vertical Projection
Mode and tap OK.
2. If you selected either
them up now. Tap Select Zone… on the Horizontal
Select Geoid… on the Vertical card of the Projection screen. You
can also wait and you will be automatically prompted to set them
before the first operation th at requires these settings.
Ground- TDS Localization
from the Survey menu. Tap the Settings button
Mapping Plane
or
for your horizontal
Geoid Model
, you can set
card and
Note: In Mapping Plane mode, when you select a zone record from
the coordinate system database, it may have a geoid model attached.
If the selected record has a geoid model attached, this record will
become the geoid for the job file and you do not need to tap
*HRLG
separately.
6HOHFW
23
GPS User’s Manual
Mapping Plane Setup
Use the Mapping Plane Setup screen to either select a map projection
zone from one of the zone groups, or select a local ized map projection
site from the database. This screen is also used to open the Projection
Key In Setup wizard where you can key in the parameters of a
custom map projection and datum.
1. Tap
2. To pick a map projection zone from the database, tap
3. If you are picking a map projection z one, select the
4. Select the
5. If you have selected a zone with a datum and a geoid model
6. Tap
6HOHFW =RQH
to open the Mapping Plane Setup screen.
in the
Data Base
the database, tap
from the drop down list.
attached, the datum and geoid name will be displayed. If you
have selected a UTM zone, you must select a
down list.
)LQLVK
record.
… on the Horizontal
box. To pick a localized map project ion site from
View Sites
or
Zone
to set the selected zone or site as the current projection
Site
in the box.
from the drop down list.
card of the Projection screen
View Zones
Zone Group
from the drop
Datum
Note: When you select a zone with no datum, after you tap
the datum selected in the drop down list will be added to a copy of the
selected zone record and you will be prompted to save the new zone
record with a unique name.
• Use the
You can only delete user created sites, you cannot delete the
original 'system' records in the database.
Note: You cannot undo the deletion of a zone.
24
'HOHWH
button to delete zones or sites from the database.
)LQLVK
,
RTK Data Collection
• Use the Key In Parameters button to open the Projection Key In
Setup screen where you can configure a custom map projection
and datum.
Projection Key In Setup
Use the Projection Key In Setup screen to create a custom map
projection and a custom datum transformation to use as the selected
mapping plane zone:
1. Tap Select Zone… on the Horizontal
to open the Mapping Plane Setup screen.
2. Tap Key In Parameters > on the Mapping Plane Setup
open the Projection Key In Setup screen.
3. Pick the
zone types are:
•
•
•
•
•
4. Pick the
datum are:
5. Select the
are:
6. Select the
projection zone. Choices for grid direction a re:
Zone Type
Transverse Mercator
Lambert 1 parallel
Lambert 2 parallel
Stereographic
Oblique Mercator Angle
Datum Type
•
Pick from Data Base
from the coordinate system database.
•
Custom Molodensky
three-parameter datum transformation.
•
Custom Similarity
seven-parameter datum transformation.
Azimuth
•
North Azimuth
grid.
•
South Azimuth
Grid
for the new map projection zone. Supported
(oblique and polar)
for the new map projection zone. Choices for
. Select this choice to use a datum
. Select this choice to enter a custom
. Select this choice to enter a custom
type for the new map projection zone. Choices
. Select this choice to have a north azimuth
. This choice to have a south azimuth grid.
direction for positive coordinates in the new map
card of the Projection screen
screen to
25
GPS User’s Manual
•
North\East Grid
increase positive in the north and east directions.
•
South\West Grid
increase positive in the south and west direct ions.
Note: The geodetic calculation engine and the Survey Pro coordinate
geometry engine are separate components. While the geodetic engine
can properly handle southwest grid systems, Survey Pro can only
operate on a northeast grid system. However, since a southwest grid
with a south azimuth is a mirror image of a northeast grid with a
north azimuth, Survey Pro can handle this configuration with the
following work around: You must set the
Azimuth
north as south and east as west, and the coordinates will be correct
for a southwest grid and south azi muth zone.
on the Units
. Select this choice to have coordinates
. Select this choice to have coordinates
Azimuth Type to North
card of the Job, Settings screen. You then treat
7. Tap
8. Key in the five or six paramet ers for your new map projection
9. If your zone is Oblique Mercator Angle, the next screen is used to
10. Tap
11. If you selected
12. Tap
13. If you are using a custom datum, enter the datum translations
26
1H[W
to enter the zone parameters for the new map
projection.
zone.
pick the
1H[W
on the datum type you specified on the first screen.
the list. If you selected either
Similarity
Ellipse from Data Base
database, or you can select
a custom ellipse.
1H[W
datum, the next screen displays the parameters for the new map
projection zone and you can
from WGS84 to Local.
and
Origin
to select the datum. The screen that opens will depend
, then select an ellipse for the new datum. You can select
to open the next screen. If you are using a database
Azimuth
Pick from Data Base
values.
, pick the database
Custom Molodensky
to use an ellipsoid record from the
Key In Ellipse
6WRUH
the record.
to input parameters for
or
Datum
Custom
from
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