[Qgis-user] Convert an ALOS PALSAR DEM from ellipsoidal to geoid.

Thu Jan 5 20:04:46 PST 2023

Hello, thank you very much for your kindness and for your time.

Happy New Year

Eddison

El mar, 3 ene 2023 a las 18:42, Nicolas Cadieux (<
njacadieux.gitlab at gmail.com>) escribió:

> 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: 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: 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
>>>>
>>>> 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
>>>>
>>>>
>>>> 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
>>>>>
>>>>> 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
>
> --
> Nicolas Cadieuxhttps://gitlab.com/njacadieux
>
>
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