[Qgis-user] Convert an ALOS PALSAR DEM from ellipsoidal to geoid.
Nicolas Cadieux
njacadieux.gitlab at gmail.com
Tue Jan 3 16:42:25 PST 2023
Hi,
I looked at vdatum and could not find how to remove an ellipsoidal.
Apparently, this software can only translate from one ellipsoid to
another but cannot simply remove the ellipsoid. I would use Ujaval's
method as this will faster to figure out then what I originally did. You
can also get help from the gdal developers.
You may use the unavco page to get more information and to double check
your results.
https://www.unavco.org/software/geodetic-utilities/geoid-height-calculator/geoid-height-calculator.html
You may also look at the GPS H tool (online and pc version) from NRCAN.
You need to create a profile if you want to download the software.
Unfortunately, this tool only works with point data but the PC version
can read geoid files from Noaa. I used that to double check my
results. To remove an ellipsoid, use GPS H using the h=H+N mode using
an input height of 100.0m.
To test (with geoid model HT2_0), for a point situated at 50°N and 100°W
if a geoid height or topographic height (H) of 100m, the ellipsoid (N)
will be at -23.434 meters and the height (h) above the ellipsoid at
76.566m. I would first try to reproduce using HT2_0 that comes with GPS
H. Then, use the EGM96 downloaded from NOAA. Finally, test using to
top left pixel of your image.
Once you transform the image using gdalwarp, look at the value of your
top left pixel to see if it matches what you get from GPS H. Warning...
this software uses GRS80 Ellipsoid. I think ALOS uses GRS80 but if it's
the WGS84 ellipsoid, you will see very small differences (probably
sub-millimeter.) I would not worry at this point.
Nicolas
On 2023-01-03 12:54 a.m., Ujaval Gandhi wrote:
> I have a blog post explaining how to do the conversion using GDAL
> https://spatialthoughts.com/2019/10/26/convert-between-orthometric-and-ellipsoidal-elevations-using-gdal
>
> On Tue, 3 Jan 2023 at 11:09 AM, Eddison Araya via QGIS-User
> <qgis-user at lists.osgeo.org> wrote:
>
> Hi, thanks, skip previous message, apparently you're right, it's emg96
>
> Thank you
>
> Eddison
>
> image.png
>
>
>
> El lun, 2 ene 2023 a las 20:08, Eddison Araya
> (<eddisonjose at gmail.com>) escribió:
>
>
> Hello yes, thank you very much, according to that source it is
> EMG-2008
>
> Regards!
>
> eddison
>
> image.png
>
>
>
> El lun, 2 ene 2023 a las 11:16, Nicolas Cadieux
> (<njacadieux.gitlab at gmail.com>) escribió:
>
> Hi,
>
> Will do. I’am heading out of the woods now. In the mean
> time, look at the metadata to find out what ellipsoid is
> being used by the file. I believe it’s probably EMG96 but
> can you confirm?
>
> Nicolas Cadieux
> https://gitlab.com/njacadieux <https://gitlab.com/njacadieux>
>
>> Le 2 janv. 2023 à 08:01, Eddison Araya
>> <eddisonjose at gmail.com> a écrit :
>>
>>
>> Hello Nicolás Cadieux, I hope you had a nice end and
>> beginning of the year!
>>
>> Ok very good, so I await your message with the step by
>> step, just in case I attach the image:
>>
>>
>> https://drive.google.com/file/d/1mJvB77MMOF86X7I4qwtS3i7UYq1wYmGR/view?usp=share_link
>> <https://drive.google.com/file/d/1mJvB77MMOF86X7I4qwtS3i7UYq1wYmGR/view?usp=share_link>
>>
>>
>> Greetings and thank you very much
>>
>> Eddison
>>
>>
>>
>>
>> El jue, 29 dic 2022 a las 21:11, Nicolas Cadieux
>> (<njacadieux.gitlab at gmail.com>) escribió:
>>
>> Hi,
>>
>> Google vdatum https://vdatum.noaa.gov/
>> <https://vdatum.noaa.gov/> Contact me early next year
>> and I can help you step by step. I can’t help you
>> more now because i’am far from civilisation with very
>> limited internet access.
>>
>> Nicolas Cadieux
>> https://gitlab.com/njacadieux
>> <https://gitlab.com/njacadieux>
>>
>>> Le 28 déc. 2022 à 09:52, Eddison Araya via QGIS-User
>>> <qgis-user at lists.osgeo.org> a écrit :
>>>
>>>
>>> Hello, a pleasure to greet you.
>>>
>>> I need to convert an ALOS PALSAR DEM from
>>> ellipsoidal to geoid.
>>>
>>> Does anyone know how to do this process step by step?
>>>
>>> Thank you
>>>
>>> Eddison
>>>
>>>
>>>
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> --
> ---
> Ujaval Gandhi
> Spatial Thoughts
> www.spatialthoughts.com <http://www.spatialthoughts.com>
>
--
Nicolas Cadieux
https://gitlab.com/njacadieux
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