Week 4

Global Positioning Systems (GPS): Differential Correction and Integration with GIS




Differential Correction allows to increase accuracy of GPS field measurement down to sub-inch or sub-centimeter by applying correction from established ground based stations also called Continuously Operating Reference Stations (CORS). These stations collect data about their locations continuously. Because CORS locations were measured by high-precision survey methods they continuously compare it with data from satellites. The difference between X and Y coordinates is stored continuously and used by Users segment to correct their own data collected in vicinity (sometimes up to few hundred miles) of the CORS.


NOAA CORS Map: https://www.ngs.noaa.gov/CORS_Map/


Differential Correction Principles: https://www.e-education.psu.edu/natureofgeoinfo/c5_p23.html


Real Time Differential Correction: https://www.e-education.psu.edu/natureofgeoinfo/c5_p24.html



Using Trimble Pathfinder Software for Differential Correction:

1. Transfer your files from Juno to the software using Data Transfer Utility.






Helpfull videohttps://www.youtube.com/watch?v=mc73j1AkwX8&list=PLZEpkLO5x2h42fXssKDTZYCCYXuklLunL&index=10


2. Start Differential Correction Utility (click Differential Correction option, see menu above) and add your .SSF (field collected and transferred) files using sign  on upper right side of the menu. See example below.



3. Click button “Next” and select Automatic carrier and Code Processing



4. Click Next button and use default settings on this:


5. Click Next button and see menu with Base Data

6. Click Select and choose a CORS closest to you with high integrity index


7. Click OK and then Next


8. Click Next and take defaults on this page:



9. Click button Start and you should see how process started



10.When process is finished you should be able to see at the end of the text sign Differential correction complete. See below.


This menu also informs you about the percentage of corrected data.


How to view field and corrected data?

Using Pathfinder Open your .SSF file (field collected). Then in File –> Background select a file for use as a background, i.e. your corrected file with the same name but extension .COR.

Go to View –> Layers –> Background, see below, and establish symbols and colors for the background data. I suggest pink color.



Then you should see both, field collected (black) and corrected data (pink).



Now you can zoom closer and use Measure tool  to see the difference in meters between your field-collected and corrected data. Measured length will be displayed in the lower left corner of the bar of the Pathfinder software.


Data Export to ArcGIS

Corrected (.COR) files can be exported to ArcGIS shapefile format using Export Utility.



After selecting Export you will see the following menu:



Click on Properties and follow these settings:


Coordinate System Tab:



NOTE:  This menu has option to Browse for Projection File (.PRJ). This is a component of the shapefile format. You can create it yourself and Browse for it here OR you can create a shapefile without it and then use Define tool in ArcToolBox in ArcMap to create it later, after you export data. Keep in mind that you need to know what spatial reference you use in your field GPS unit. In the example above it is a regular Geographic Coordinate System. This is what I usually do when setting up GPS field device.


Data Tab:



Position Filter Tab:


NOTE: These settings in Position Filter Tab will export only corrected data. However, if you have uncorrected data in your file you will need to check also “Uncorrected” box.


Attributes Tab:


Units Tab:


Now you are ready to export. Click OK and then specify output location of your shapefile. In this case it is C:\yuri\gis_misc\Export




Managing Exported Shapefile in ArcGIS:


1. If you did not create in advance, create .PRJ file using Define tool in ArcMap ArcToolBox.


2. When you created .prj file I recommend to export your shapefile “Point_ge.shp” into something more meaningful, e.g. “orchard_beach_survey2017.shp” or similar.


3. Do not forget to go to View à Data Frame Properties à Coordinate System and set up your spatial reference. You can choose it there OR you can Import it form just updated shapefile “Point_ge.shp” or exported shapefile with different name.


4. Open attribute table of your shapefile and make sure that everything you need is there. If not, go back to Export Utility in Pathfinder and change export settings.

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