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Extracting Points from Base & Rover GCP survey

(Chelsea Volpano) #1

Hey all-

This is my first experience with GPS surveying and I don’t think it’s gone well. I used 2 Reach units (base & rover) to collect the location of GCP’s for a drone survey of an area, went back a month later and did the same thing.
I am looking to find elevation difference in the DEM’s but they are 5-10m in height and I should only be seeing centimeters of change (although they seem to be laterally accurate).
Obviously I must have done something pretty wrong, as I said I was navigating this process rather blindly, I am hoping that someone could help me start from the beginning and salvage some usable data.

-I did not have the correct settings for the first survey and no longer have access to the units, are they recorded in any of the output files?

-Using RTKpost I entered the .obs files for the base & rover and the .nav for the base. I have what looks to be an accurate Ground Track when plotted as a KML in Google Earth, but how do I find only the data for Points I collected during the survey?

I would really appreciate any input or constructive scolding…

(Tobias Dahms) #2
  1. The nav file should be for the rover.
  2. If you have an export of the survey e.g. a csv you can find there the begin and end of the survey.
  3. Height differences might be due to wrong processing (s. 1.) or due to changed processing options, i.e. ellipsoid vs. geoid height.
(Chelsea Volpano) #4

  1. On the Emlid Post Processing Tutorial doc it says “Put base or rover .nav file in the third field.”. How do you know to use one vs. the other?

  2. I have csv’s with my points but the elevations in those are also not correct, which is why I was thinking that my initial log settings might have been incorrect.

There is a CORS reference station about 14km away from my survey area, I processed my Base data vs the CORS station to get a corrected absolute position for the base. I input that position into the Base Station Position but the result was a really noisy ground track with mostly FLOAT. Would this be the correct way to use a reference station?

I’m not understanding how you know what settings to use. Do you post process using the settings applied during the survey? Or do you set Options to what you are looking to get from the data?

Lastly, assuming I eventually get accurate corrections: Would I use the time stamps from the .csv to extract the corrected survey points?

(Tobias Dahms) #5
  1. you are right, the nav file should be picked from the unit with the best sky view.
  2. I think the csv-file elevations are influenced by antenna height etc. I always do post processing to get a better idea about the quality of the results.

I also sometimes use an external reference station to determine the position of my base. I calculate the static position of my base with the Integer Ambiguity Res setting “continous”. I recalculate the position for diffenrent observation periods and then I choose the results which have a high fix ratio and take the mean of those results as the position.

The base station position should have no significant impact on the quality of your rover track.

I would use the the observation periode given for each point in the csv file to recalculate the position in RTKpost. If the observation time is long enough for the given base/rover distance I would use the static mode.