High end stuff. I never trust any consumer grade GPS device altitude readings. I have seen them off by as much as 300 to 400 feet. High end GPS devices rely on either a real time data link to ground reference stations or they post process the data back at the office. This is a process where you time sync your data with the ground reference stations, calculate the timing difference based on your location distance from the ground control points. The ground control points have a known coordinate and altitude. They then compare the GPS readings at their point for those time reference points you process against. They calculate the correction factor from the GPS against the known point, then also correct your data based on the distance/time from the ground station and correct your data. For older GPS units you sometimes had to occupy a point for a longer period of time based on your distances from the control points.
High end GPS units can also do RTK or real time kinematic data collection. This is done by either having a GPS base station controller that is real time data linked to ground control stations or that the rover unit has the real time data connectivity. They do all the above processing steps in real time, no post processing after the field work is done. Depending on the equipment and setup, there are time requirements to have the instrument on point to achieve the desired level of precision. Depending on the type of work, faster systems are desired when you are doing things like having to stand in traffic to map every manhole and gas valve. Other uses that are slower can be fine if you can leave it on a tri-pod or bi-pod for 30 seconds or longer. Slower systems are preferred by workers getting paid prevailing wages ;-) Thank you, Brian Webster www.wirelessmapping.com From: AF [mailto:af-boun...@af.afmug.com] On Behalf Of Josh Luthman Sent: Wednesday, May 6, 2020 11:17 AM To: AnimalFarm Microwave Users Group Subject: Re: [AFMUG] Elevation question >local Geoid data set loaded Is this something most GPS radios would have? Or just higher end ones? Josh Luthman Office: 937-552-2340 Direct: 937-552-2343 1100 Wayne St Suite 1337 Troy, OH 45373 On Wed, May 6, 2020 at 10:36 AM Brian Webster <i...@wirelessmapping.com> wrote: OK here is what I learned from my brother. You want to use the Geoid values, however you also want to make sure you have the Geoid model loaded in to your Trimble R8 GPS. (for those not paying attention that is a survey grade instrument with cm accuracy). The geoid numbers are tied to the map projection you are using and they are also tied to the GPS ground stations you are connected to 9necessary for the cm grade accuracy). The Ellipsoid values assume that the earth is perfectly/mathematically round. Geoid values correct for the fact that it is not perfectly round and the reason why your GPS needs the local Geoid data set loaded before you take readings. You do have the Geoid data for your area loaded don’t you? For radio mobile you will want to use the Geoid height converted to meters. Is there LIDAR data for the area in question? It might be easier just to use a value from that. Might also be nice to convert the LIDAR to a file format for Radio Mobile and use all that data (going to need a lot of disk space though). Thank you, Brian Webster www.wirelessmapping.com From: AF [mailto:af-boun...@af.afmug.com] On Behalf Of ch...@wbmfg.com Sent: Tuesday, May 5, 2020 2:12 PM To: af@af.afmug.com Subject: [AFMUG] Elevation question I have a pretty good GPS receiver. Trimble R8 with a Yuma data collector. Normally we use it only for lats and longs. It is corrected in real time with a cellular data modem and a RTK/VRS ground station network. Yesterday we did a shot from a hill that did not exist when shuttle radar data was taken. It was way off in elevation. So we took a shot from our parking lot and compared it with what we think we know to be the altitude as well as our cell phones and google earth. Turns out the R8 has an ellipsoid height as well as another height associated with the northing and easting values. The second one seems to be more in line with what I was expecting. The local dealer sent me a drawing showing this. (attached) So now I am super confused. Not sure what number to use when working with Radio Mobile. Do I add the geoid height to the ellipsoid height? Was hoping Brian Webster may know or others may know. Googling google earth elevation talks about different versions of geoid. Not sure if their elevations are geoid elevations or not. So, rather than dig into it any more I am taking the lazy approach and posting the question here. -- AF mailing list AF@af.afmug.com http://af.afmug.com/mailman/listinfo/af_af.afmug.com
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