[Therion] 3D model height

[Benedikt Hallinger]

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?
> 
>  		 [2]
>  		Mentes a vírusoktól. www.avg.com [2]
> 
> On Fri, Feb 19, 2021 at 9:18 AM Benedikt Hallinger
> <beni at 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.
>>> 
>>> [1]
>>> Mentes a vírusoktól. www.avg.com [1] [1]
>>> 
>>> 
>>> 
>>> Links:
>>> ------
>>> [1]
>>> 
>> 
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