Post Processing GPS Data

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Post Processing GPS Data

• Transfer data from GPS to computer using
  ActiveSync Windows Explorer
• Check-in the GPS data to the geodatabase using
  GPSAnalyst in ArcMap
• View the GPS data in ArcMap
• Differentially correct the GPS data (includes
  Rebuild the features after differential
  correction)
• Review and edit the data

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1. Transfer data from GPS to computer
Connect the cables and place the GPS in the
cradle. Select the Explore button from the
ActiveSync dialogue. This will launch the
Windows Explorer on the GeoXT. Once open, copy
and paste the folder containing the ArcPad .apm
files from the GeoXT to the PC. Be sure to place
the checked out folder from the GeoXT containing
the edits in the same location on the PC as it
was copied from. Accept any data formatting
dialogues that may appear during the transfer by
selecting OK. The folder and files containing
the data should now be transferred to the PC.
When this is done you should be asked if you
would like to overwrite the existing folder and
all of its contents. Select Yes to All.
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2. Check-in the GPS data to the geodatabase
Launch an ArcMap session and Open the .mxd
project file which contains the data you
originally checked out. (Or you can also add the
data layers that were checked out if you did not
save them as a .mxd project). To edit features
collected with GPS go to the GPSAnalyst toolbar
and start a GPS edit session, not a regular
ArcMap edit session (in a regular edit session
the features will become disassociated from the
GPS locational data). You can only edit GPS data
in a GPS edit session you cannot edit other
types of data. Next use the Check In ArcPad
Shapefiles GPSCorrect SSF Files button on the
GPS Analyst toolbar when you check in features
collected with ArcPad and GPScorrect. Us of this
tool assures that the ssf data accompanies the
features. (The features are imported into the
feature classes they were checked out from. The
ssf data are stored in an imported GPS session in
the GPS Sessions folder of the geodatabase). The
associated dialogue lists all the layers in the
folder. Select those you want to check back into
the geodatabase and click the check in button
at the bottom of the dialogue.
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2. Check-in the GPS data to the
geodatabase(continued)
Close the Check In dialogues. The new/updated
data is now checked in. The corresponding ssf
file is imported into the geodatabase as an
imported GPS session and the GPS data is linked
to the GIS features. From the GPS Analyst
dropdown menu select Save Edits. This saves the
modified features and the associated GPS data in
the geodatabase. From the GPS Analyst dropdown
menu select Stop GPS Editing. Select File/Save
to save the map document. The raw GPS data is
now viewable in ArcMap.
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3. Differentially correct the GPS data
From the GPSAnalyst drop-down menu, select Start
GPS Editing. Click the differential correction
button on the GPSAnalyst toolbar and the wizard
appears. The Select GPS Sessions to correct list
is either empty or displays a list from previous
work. Remove any listed sessions by selecting
them and clicking the Remove button in the
dialogue. To select the GPS session that was
created when GPS data was imported into this map
click the Add GPS sessions button. Now the
Select GPS sessions to correct dialogue appears.
Browse to the correct GPS Sessions folder, select
the GPS session that was created when the GPS
correct SSF file was checked in. Click Add. The
dialogue will close and the selected GPS session
appears in the Select GPS sessions to correct
list in the Differential Correction wizard. Click
Next to get to the Processing Type page. Enable
Standard Code Processing Only.
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3. Differentially correct the GPS data (continued
1)
Click Next to get to the Correction Settings page
and customize differential correction. Standard
is fastest. Velocity filtering should be used
when the data was collected in a high multipath
environment or while moving rapidly along a line
or polygon feature. You can also set new
elevation, SNR and DOP filters for your rover
data, but this should not be necessary. Instead
select the Use Data Collection Filter setting
option to use the filters set in GPS Correct (in
ArcPad). In the Output tab, select an option from
the Output Positions group, which determines the
data to be sent. Typically corrected only is
sufficient, but it can be informative to get both
corrected and uncorrected to see the effects of
differential correction. Select an option from
the Audit File Contents group specifying what is
written to the log file. Typically standard is
sufficient to FGDC metadata level information. In
the Base tab, select a Base Data Processing
Technique. For now, experiment with standard,
filtering and smoothing to see the effect of
rejecting outliers, etc. In the Base tab, select
filters for the data coming from the base station
and used for differential correction minimum
elevation (10), minimum SNR (6), These are
similar to the elevation and SNR filters on your
rover data but in this case apply to the base
data. Click OK to save differential correction
settings.
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3. Differentially correct the GPS data (continued
3)
Click Next and the select Base Data page appears.
Options are to download base data from the
Internet or use base data from a folder on the
hard drive. We will use the Internet
option. Reference Position Settings This refers
to the correct coordinate location of the base
station and it is very important to get this
right! Remember that the entire differential
correction calculation depends on having the
correct location for the base station. The best
practice is to use the second option, use
reference location from the base provider. This
is the surveyed location determined when the base
station was established. The first option
listed, use reference location from base files
is the first position recorded by the base
station in the hourly file you are using. This
coordinate location may vary and should be used
only if you have reason to believe the surveyed
location is incorrect. (e.g. if you know there
was an earthquake that affected
Woodward). Confirm your settings before
postprocessing. Select the check box to confirm
the coordinates of the reference position and the
availability of the base files to be used. This
information appears in the last page of the
wizard before you start the differential
correction. If the check box is NOT selected,
clicking Start on the last page of the
differential correction wizard will automatically
start the process without confirming that the
base data is available (possibly resulting in no
correction!).
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3. Differentially Correct GPS Data (continued 4)
Click Next and the Rebuild Settings page of the
wizard appears. It displays the settings that
GPSAnalyst will use to rebuild each feature
linked to GPS data. The default settings should
be used. These are 1. Use the most recent
positions from all GPS sessions 2. Filter by
correction type 3. Do not filter by position
quality 4. Manually edited geometries will be
replaced with GPS geometries. Click Start and the
differential correction process starts. The next
page of the wizard shows the status and progress
of each process. A completion message is
displayed when finished, including estimated
accuracy values. Click Close. The contents of
the completion message are saved as the
Differential Correction report stored in the Log
Files folder in the same folder as the open
geodatabase.
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4. Review and Edit the Data
Recall the difference between GPS features and
GPS positions. To see the many positions that
are averaged to create a single point there are
several options. You can display only certain
types of GPS positions or all of them. See the
GPS Analyst Help for more information on
this. You can select features in ArcMap and then
click one of the GPS positions buttons on the
GPSAnalyst toolbar to display the positions for
those features. To view detailed information
about positions use the GPS Position Properties
button in the GPSAnalyst toolbar.
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Editing
Do it ASAP, before collecting more data, if at
all possible. Scrutinize before you edit, check
your memory and your field notes before you
edit. Keep a copy of the original unedited
file! Check features, attributes and spatial
pattern.
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Editing
Point Features Location Is the point misplaced
relative to other features or an expected
pattern? Check the corrected positions that the
point is based on. Check field notes for
constellation jumps, PDOP spikes. Point
Features Theme Is the feature correct? Are the
attributes correct?
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Editing
Line and Area Features Location Are features in
the right location relative to each
other? Are straight lines straight? Are
curves and corners fully spatially defined
with enough vertices? Are there too many
vertices? Do any arcs cross? Line and Area
Features Themes Is the feature correct? Are
the attributes correct?
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Line and Area Editing