If I were to enter the ellisoid height, the error of Proj4 would still remain on the EOV projection. I could work in a UTM coordinate system, but it would be much simpler if Therion didn’t recalculate anything. (There is no height issue between UTM and WGS84 because there is no conversion) This error is only present in the national projections where Proj4 performs approximate calculations for the advantage of horizontal coordinates and the disadvantage of height. On Fri, Feb 19, 2021 at 11:25 AM Balambér Hakapesz <holl.balazs63@gmail.com> wrote:
thconfig: #cs long-lat #fix 0 19.009617 47.526453 262 cs EPSG:23700 # Hungarian EOV fix 0 647151 242509 262
kml: <LineString><coordinates> 19.00961011626948,47.52649337576407,291.08568208889801
291-262 = 29 m = 44m geoid height - 15m Proj4 error
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On Fri, Feb 19, 2021 at 11:06 AM Benedikt Hallinger <beni@hallinger.org> wrote:
Isn't EGM84 the reference ellipsoid currently used by WGS84? What you get with GPS is *not* sea level, but the altitude above that reference geoid (which takes into account gravital anonalys).
If you calibrate your barometric altimeter to the hieght given by a height coordinate of WGS84 (like read from your car's navi at the parking lot, or from a map calibrated to WGS84) then you should get about the same altitude. If you take the cave location from GPS and enter that coordinates into the therion dataset wirh correct "cs" marking, the conversion should yield the correct result. If you take the altitude from a map (or an altimeter calibrated to that map) in another coordinate system, you have to either apply an offset manually, add an instrument correction factor in therion, or supply the correct coordinate system (which then probalby demands conversion of the lat/lon coordinates).
If generating 3D surfce meshes, you also need to supply matching coordinate systems data and/or specify the correct "cs" parameter.
I think your problem comes from mixing coordinate systems in respect to the height model, either at data input or when using the height mesh.
Am 2021-02-19 9:49, schrieb Balambér Hakapesz:
Very rarely can anyone work at ellipsoidal heights. In local coordinate systems, it only makes sense for upper geodetic calculations. Only for GPS measurements we get an ellipsoidal altitude, even there the devices can immediately calculate the altitude of the EGM96 model with an altitude above sea level (with a few meters error which is not significant compared to the 15m altitude error of handheld GPS). Proj4 calculates the horizontal coordinates well, but there is a serious error in the height coordinates (for special national projections (http://www.agt.bme.hu/gis/workshop4/eloadasok/proj_poszter_3d.pdf)), so it would be better if not we would count on it but leave the altitude unchanged. Could it be an option not to change the height with Therion?
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On Fri, Feb 19, 2021 at 9:18 AM Benedikt Hallinger <beni@hallinger.org> wrote:
Hi there, if you measure the altitude with a barometric device, you do not get the altitude above the ellipsoid. You have to account for that at the input of your data.
With the "cs" command you define your input data to be in a specific
coordinate system, and also your altitude has to be given in that system. If you just put the barometric altitude there (which you usually did calibrate to a map, didn't you?) you have to transform the altitude manually if the reference altitude differs.
Am 2021-02-19 8:44, schrieb Balambér Hakapesz:
Hi!
When generating 3D models, Proj4 transforms between different coordinate systems. This also transforms the ellipsoidal altitude, but the data always refer to altitude above sea level and should not be changed. The finished models do not fit the topography, you have to cheat with the height to be good.
Balázs Holl.
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