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
<[email protected]> 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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