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How Accurate are GPS Units for Topographic Surveys

Postby Bobi73 » 08 May 2009 21:32

Hey Guys

I'm planning on designing a couple of simple gravity fed water systems and was wondering if anyone had any information on the levels of vertical accuracy I can expect from a GPS unit. I'm using a Garmin Etrex Vista that has an altimeter.

Thanks

Bobi

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Re: How Accurate are GPS Units for Topographic Surveys

Postby MichaelHawkins » 13 Jun 2009 14:51

Dear Bobi

I recall reading that the vertical GPS accuracy was twice the horizontal accuracy (as stated in real time on the GPS unit) i.e. if the GPS is giving the position to within +/- 6m horizontal accuracy it should be +/- 12m on the vertical accuracy. I can't recall the source but I'll see if I can dig it up. Either way, GPS units aren't teribbly accurate for topographic surveying, even now that the selective availability signal has been turned off. You may get better results using the pressure altimeter (which I believe are accurate to +/- 2m on the Etrex Vista, although again I'll need to track down the source on that one). You'll need to use a differential GPS approach (i.e have one static and one mobile station) to counter the effect of pressure variation (which from my experience can be significant).

Good Luck

Michael

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Re: How Accurate are GPS Units for Topographic Surveys

Postby Gazman » 03 Jan 2010 22:18

Bobi
Chapter 5 of the Field Guide to Environmental Engineering for Development Workers covers topographic surveying for water systems using GPS units equipped with altimeters. On page 88 there is an interesting graph that shows the range in pressure variations for a static location throughout the day. As you can see the atmospheric pressure can throw off your topographic survey results for up to 35 meters! This is significant if you are surveying for a water network or spring line!

altimeter.jpg

The chapter goes on to describe how to compensate by using two altimeters (one static and one roving) and ajusting for pressure variations. GPSUtility is also mentioned which is an excellent tool for pulling altimeter tracklog data from a GPS unit and importing into Excel for topographic survey analysis in case people haven't come across it.

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Re: How Accurate are GPS Units for Topographic Surveys

Postby CatStevens » 07 Jan 2010 14:06

According to Garmin my etrex Summit is accurate to within 10 feet. It's has a pretty sensitive atmospheric pressue sensor (there's a tiny hole on the back), if you lift it up to eye level from floor level it registers a corresponding increase in elevation of a couple of feet. Of course it is also pretty sensitive to atmospheric weather variations and needs to be corrected as described by Gazman.

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Re: How Accurate are GPS Units for Topographic Surveys

Postby Sebastian » 09 Jan 2010 11:24

The International Rescue Committee put together this guide to GPS Surveying Procedures which describes the altitude correction method using two gps altimeter units. One thing I have found that is not mentioned in the instructions is that before you begin you have to turn the altimeter autocalibration setting off in the options mennu otherwise the unit will try to correct the altitude based on GPS signals. In addition, there is now a more recent version of GPS Utility available http://www.gpsu.co.uk/ that allows tracklog and waypoint output directly to Google Earth with very little complication.

GPS Surveying and Plotting Procedures.doc
(82 KiB) Downloaded 453 times

Surveying and Plotting Procedures
using e-Trex GPS Vista and GPS Utility 4.0 Software

1. Synchronize clocks in GPS and Pressure Altimeter (PA).
2. Place one person with PA at mid-point of line to be surveyed (in order to adjust for effects of weather variations on altimeter readings). Alternately, if you have two PAs you can place one PA at the start point (PASP) and one PA at the end point (PAEP).
3. Have person(s) with PA write down Time and Altitude every five (5) minutes throughout the survey.
4. GPS surveyor goes to start point of the survey.
5. Turn on GPS and wait until satellite reception is OK and accuracy is less than 10 meters.
6. Calibrate altimeter (if desired).
7. Calibrate compass.
8. Go to Main Menu and Tracks
• Ensure Track Log is ON.
• If you have already downloaded previous data, Clear track log.
• If you have already downloaded previous data, go to the Option Menu and Delete All Saved.
• Go to the Option Menu and select Setup Track Log. Check the Wrap option, specify track Record Method (recommend Auto) and specify Interval (recommend Normal).
9. Go to the Trip Computer page and Option Menu. Click on Reset. Highlight Select All, then Apply and then “OK”. This will delete all routes, tracks and waypoints stored in the GPS. If you have not downloaded these data sets or if you want them to remain in the GPS, do not delete. The purpose of deleting previous data is to eliminate confusion between earlier data and the data you are about to collect during the survey.
10. Name (Mark) the start point.
11. Begin walking the pipeline.
12. Name (Mark) key waypoints along the survey route.
13. Turn off GPS at completion of walking entire survey line.
14. Collect the PA data record(s).
15. Turn on and connect the GPS to your computer using the PC interface cable.
16. Double-click on the GPSU 4.04 icon.
17. Select GPS and Download All.
18. Save this file as a text file and name it appropriately (e.g., StyaGPS.txt).
19. Disconnect the PC interface cable and turn off the GPS.
20. At this point you can select File and Open the saved text file.
21. In the Waypoint data file, select the appropriate waypoints, in sequence from start point to end point, and create a new route which portrays the pipeline surveyed. Do this by highlighting the first waypoint, selecting Tools and Insert in Route. Continue for the second and subsequent waypoints. Upon completion, this route will appear on the map.
22. At this point, select File and Save in order to save the route created.
23. View the map, waypoints, tracks, track summary, routes and other data in GPSU 4.04 as you wish.
24. When you are ready to plot the track and route profiles, you will need to save the files in a format compatible with Excel. For the track profile, select File, Save/Print Options, and save as a comma separated file. Give this file a different name than the original file (e.g. StyaTrack.txt). For the route profile, select File, Save/Print Options, change the format to “Route details (tab separated file)” and save as a comma separated file. Give this file a different name as the original file (e.g., StyaRoute.txt).
25. Exit out of GPSU 4.04 and then open the newly saved track text file (StyaTrack.txt) in Notepad and use the text editor to delete all rows except those rows containing track data. This is necessary to ensure that Excel can separate the track data by columns.
26. Save the file and close it.
27. Open Excel and then select File, Open and specify the file you just edited (e.g., StyaTrack.txt).
28. Use the Text Import Wizard and select “Delimited”, “Next”, “Comma”, “Next” and then “Finish.”
29. Delete all columns except the Altitude, Time and Distance.
30. If you used one PA, then:
• organize six (6) columns in Excel in the order of:
TIME PA PA ALTITUDE DISTANCE CORRECTED ALTITUDE
• Enter the data recorded from the PA next to the correlated time in the TIME column.
• Calculate PA as follows: PA = PA – PAbaseline. Where PAbaseline is the PA recorded at the starting time (i.e., the first entry in the PA column).
• CORRECTED ALTITUDE = ALTITUDE - PA
31. If you used two PAs, then organize eight (8) columns in Excel in the order of:
TIME PASP PASP PAEP PAEP ALTITUDE DISTANCE CORRECTED ALTITUDE
• Enter the data recorded from the PASP and PAEP next to the correlated time in the TIME column.
• PASP = PASP – PASPbaseline. Where PASPbaseline is the PASP recorded at the starting time (i.e., the first entry in the PASP column).
• PAEP = PAEP – PAEPbaseline. Where PAEPbaseline is the PAEP recorded at the starting time (i.e., the first entry in the PAEP column).
• CORRECTED ALTITUDE = ALTITUDE – [PASP - (PASP - PAEP)(DISTANCE/TOTDIST)]
o Where TOTDIST is the total distance from the start point to the end point (i.e., the last entry in the DISTANCE column).
32. Highlight the Distance and Corrected Altitude columns.
33. Select “Insert”, “Chart”, “Standard Type”, “XY (Scatter)”, “Next”, “Data Range”, “Next”, and then specify the names for the Chart Title and the Units of Measure for the distance (x-axis) and elevation (y-axis).
34. Select “Next” and then “Finish.”
35. The profile will now appear in the Excel file. You can now modify the chart as you wish.
36. When finished, be sure to save this file as an Excel file (e.g., StyaTrack.xls)
37. Open the route text file (StyaRoute.txt) in Notepad and use the text editor to delete all rows except those rows containing route data.
38. Save the file and close it.
39. Open Excel and then select File, Open and specify the file you just edited (e.g., StyaRoute.txt).
40. Use the Text Import Wizard and select “Delimited”, “Next”, “Comma”, “Next” and then “Finish.”
41. Delete all columns except the Waypoint ID column (column A), cumulative distance column (column E) and the time (column F).
42. Create a 4th column titled Altitude. You will have to manually enter the altitudes in the Excel spreadsheet at this point. Open up GPSU 4.04 and open the GPS data file (StyaGPS.txt). Click on the Waypoints page. Now read off the altitude for each of the waypoints in your spreadsheet and type the appropriate altitude in the Altitude column.
43. Close GPSU 4.04.
44. If you used one PA, then:
• organize seven (7) columns in Excel in the order of:
ID TIME PA PA ALTITUDE DISTANCE CORRECTED ALTITUDE
• Enter the data recorded from the PA next to the correlated time in the TIME column.
• Calculate PA as follows: PA = PA – PAbaseline. Where PAbaseline is the PA recorded at the starting time (i.e., the first entry in the PA column).
• CORRECTED ALTITUDE = ALTITUDE - PA
45. If you used two PAs, then organize nine (9) columns in Excel in the order of:
ID TIME PASP PASP PAEP PAEP ALTITUDE DISTANCE CORRECTED ALTITUDE
• Enter the data recorded from the PASP and PAEP next to the correlated time in the TIME column.
• PASP = PASP – PASPbaseline. Where PASPbaseline is the PASP recorded at the starting time (i.e., the first entry in the PASP column).
• PAEP = PAEP – PAEPbaseline. Where PAEPbaseline is the PAEP recorded at the starting time (i.e., the first entry in the PAEP column).
• CORRECTED ALTITUDE = ALTITUDE – [PASP - (PASP - PAEP)(DISTANCE/TOTDIST)]
o Where TOTDIST is the total distance from the start point to the end point (i.e., the last entry in the DISTANCE column).
46. Highlight the Distance and Corrected Altitude columns.
47. Select “Insert”, “Chart”, “Standard Type”, “XY (Scatter)”, “Next”, “Data Range”, “Next”, and then specify the names for the Chart Title and the Units of Measure for the distance (x-axis) and elevation (y-axis).
48. Select “Next” and then “Finish.”
49. The profile will now appear in the Excel file. You can now modify the chart as you wish.
50. When finished, be sure to save this file as an Excel file (e.g., StyaRoute.xls)

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Re: How Accurate are GPS Units for Topographic Surveys

Postby mahmah » 14 Sep 2010 04:51

Hi all,
i really get intrested on this issues,
ia have on my work the same proplem ... i try to make topography survy,
i take all of my readings during the summer while the humedity was very high and the wether was very hot,,, and i make comparison for some selected readings with google earth , but i find different between the two readings,
even when i try to place the points on ARCGIS-9.3 i have some proplems !!
another thing :- how i can make coorection for the position according to the accuracy i have from the GPS??
all the best

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Re: How Accurate are GPS Units for Topographic Surveys

Postby Joyce » 05 Oct 2010 04:05

mahmah. It doesn't matter if the weather is warm and wet, or cold and wet, or warm or dry! The most important thing is that the pressure doesn't change during the course of the topographical study. The process that Sebastian describes (using two GPS units - i.e. a mobile and a base unit) allows you to take into account these pressure changes and correct for pressure changes during the course of the topographical study. A good principle in topographical surveys is to try to "close the loop" i.e. always try to go back to where you started to see if your device gives you exactly the same altitude.

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