patiam
Core User of DJI
Flight distance : 1118740 ft
United States
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Mark Steggs Posted at 5-11 05:46
Hello and thanks so much for sharing your information. Us newbies (I'm 2 months into the aerial world) really appreciate it and hope to return the favour some day!!!! Just a quick question, can you suggest a method that I can use to determine the height above ellipsoid? I note you convert the images to your local datum and then shift to your geoid12B by inputting -34.186. How do you arrive at this figure? I have a GPS Topcon base station and rover and am learning to use it to log raw data and post process using KLAU software. Can I measure this easily with my gear do you think or is there somewhere I can go to online? Appreciate all your commentary!!! Mark.
You can find your Geoid12B height either way (using your gear or with an online service). The easiest is just to go to the NGS website and input your Lat/Lon (You don't need super accurate coordinates, the Geoid doesn't vary much on a scale of 10's of meters - you can test this yourself by inputing coordinates near each other).
This is from another thread:
...The difference between Ellipsoid & NAVD88 Orthometric Height defined by GEOID12B (or any other Geoid Model) varies over space. This is because the geoid surface is irregular, unlike the reference ellipsoid. So you need to find the value that is appropriate for your site (and if your site is quite large, and you want the highest accuracy possible, you should not apply a single correction but one that varies across the area mapped - that can be accomplished a number of ways).
You can get the value for your base station location from a number of sources, and perhaps the National Geodetic Survey (NGS- part of NOAA, they run OPUS as well) is the best one-stop shop:
- If you're using a known published benchmark the Ellipsoid and Orthometric Height should be in the published station info (as in HTA_RP-1's example). If it's a CORS station, NGS will have it (as well as data from the station if needed for PPK processing, etc.)
- The easiest way to get the value for an known but unpublished location is also at NGS, where they have an interactive toolinto which you can enter Lat/Lon for a single site or a file of multiple sites, which will return the GEOID12B height. While the GEOID12B height varies over space as noted above, it does so on a scale that even the error associated with a decent autonomous GPS Lat/Lon position (3-5m) will still give you an accurate value in most locations. They also have software tools that run in Windows or Unix that you can download and run locally (with downloaded data for your area)
- If you need the value for an unknown location, and you are going to use your RTK GPS to site it in as a new benchmark, you can record data for at least 15min but not more than 24hrs in a RINEX file, and upload it to OPUS (Online Positioning User Service) for processing. You have to wait until the next day as most CORS stations roll over their files every 24hours, and OPUS needs their data to process your file. You'll need to enter the antenna model and height there when you upload the file. OPUS will either rapid-static or static process the data using a network of nearby CORS stations and send you a report like the one I pasted in my post (I left off the section lising all the stations used and their info). The report will have your previously unknown benchmark coords in earth-centric (XYZ) coordinates for both the NAD83 and IGS08 datums, as well as Lat/Lon in DMS and the Ellipsoid height for both those datums, as well as the NAVD88 Ortho height for NAD 83.
Lots of info on how OPUS works and how to interpret the report is here. Your blue square indicates the Ellipsoid and Ortho Heights in m. The red square shows the error of those values (standard deviation for rapid-static, peak-to-peak for static); this is the case for the Lat/Lon's listed above and all XYZ & LAT/Lon positions in the report. Next to every position is it's associated error and the units of the error (yes, the EL HGT error is just 9mm, and while the ORTHO HGT is a bit worse at 24mm, due to the uncertainty in the GEOID12B model, but still... That's pretty dead nutz!).
- If you need to apply the GEOID12B model over a larger area you can download it for different areas and in various formats from the main GEOID12b page, CONUS is here. The model can be applied in some processing software (not yet in Pix4D unfortunately), or you can leave your Pix4D DSM/DEM/XYZ outputs in HAE and apply the GEOID12B model in GIS or elsewhere. Note that the resolution of the downloaded file is 1 arc-minute (about 31 m or 101 ft at the equator), so you won't see any variarion of the model at scales smaller than this. If you need finer resolution then you need to use the interactive tool on the website or the downloadable one to generate heights at dicrete locations, which you can then apply
Lots more info on GEOID12B here
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