[GRASS-user] Unable to perform resampling
Veronica Andreo
veroandreo at gmail.com
Mon Nov 23 15:40:33 PST 2020
Well, yes, there's the mailing list, posts with questions in
https://gis.stackexchange.com, twitter account and facebook community, and
the issues in the GitHub repo for reporting bugs or proposing new features.
Moreover, you can check https://grass.osgeo.org/learn/ for plenty of other
resources (tutorials, courses, books, etc.) and the wiki (
https://grasswiki.osgeo.org/wiki/GRASS-Wiki).
I believe mailing lists work better for most of us. Also, that's where you
can reach a larger audience. Having to check yet another platform to see if
there are questions that are mostly already answered in the mailing list
archives, tutorials or the wiki seems like a bit too much to me. However,
if someone else is willing to manage such a forum, they are most welcome :)
best,
Vero
El lun, 23 nov 2020 a las 23:22, BHARATH RAM (<br11 at iitbbs.ac.in>) escribió:
> Okay, thanks for the clarification.
> (Also, I will remember to cc the list in future).
>
> I have one more question if it's okay to ask(not related to above issue).
> I understand Grass GIS queries & issues are resolved through mailing lists
> at the moment and the archives. Not meaning to be disrespectful of the
> community, but won't it be much useful when we have a forum like website to
> post & answer each others queries (it can especially be useful since
> newcomers like me don't have to flood the mailing list with common issues)?
>
> Thank you.
>
> Regards,
> Bharath Ram.
>
> On Tue, Nov 24, 2020, 3:30 AM Veronica Andreo <veroandreo at gmail.com>
> wrote:
>
>> [cc-ing the list]
>>
>> The examples in the manuals are in meters because they are based on the
>> North Carolina sample dataset which CRS is Lambert Conformal Conic. The
>> units of this CRS are meters.
>>
>> g.proj -p
>> -PROJ_INFO-------------------------------------------------
>> name : Lambert Conformal Conic
>> proj : lcc
>> datum : nad83
>> a : 6378137.0
>> es : 0.006694380022900787
>> lat_1 : 36.16666666666666
>> lat_2 : 34.33333333333334
>> lat_0 : 33.75
>> lon_0 : -79
>> x_0 : 609601.22
>> y_0 : 0
>> no_defs : defined
>> -PROJ_UNITS------------------------------------------------
>> unit : Meter
>> units : Meters
>> meters : 1
>>
>> You have to adjust to the units of the CRS that you use when it comes to
>> resolution. There's no way to set resolution in meters if you work in
>> latlong. Hence the recommendation to work in projected systems.
>>
>> Some modules might use distance in meters for certain parameters even
>> when working in latlong, because they convert such distances within the
>> source code into radians (if I'm not mistaken). If not, they will raise a
>> warning. Again, to avoid such issues, it's a good practice to work with
>> projected CRS.
>>
>> best,
>> Vero
>>
>> El lun, 23 nov 2020 a las 22:46, BHARATH RAM (<br11 at iitbbs.ac.in>)
>> escribió:
>>
>>> Dear Ms.Veronica Andreo,
>>> Thank you very much for your quick reply. Yes your suggestion works
>>> correctly.
>>>
>>> But I find it a litttle suprising since in the documentation (for
>>> modukes like r.resample.stats, r.sun etc..) & in several examples meter
>>> seems to be the default units used for distance and I assumed that for my
>>> case as well it was the same (even though g.region -p prints out in decimal
>>> degree).
>>>
>>> Is there any way to change the units back to meter as default?
>>>
>>> Thank you
>>> Regards,
>>> Bharath
>>>
>>> On Tue, Nov 24, 2020, 3:03 AM Veronica Andreo <veroandreo at gmail.com>
>>> wrote:
>>>
>>>> Hello Bharath,
>>>>
>>>> I think your problem is that you are working in a latlong location and
>>>> trying to set a resolution of 8000 degrees, which is not possible. If your
>>>> desired final resolution is ~8 km, you will need to set the region
>>>> resolution to its equivalent in degrees (the unit of latlong CRS).
>>>> Otherwise, work with projected CRS like UTM for example (recommended).
>>>>
>>>> Furthermore, if you want to go from high res to lower res (upscaling),
>>>> r.resamp.stats is your friend, so the second approach would be fine. The
>>>> problem lays in the resolution you are setting.
>>>>
>>>> HTH,
>>>> Vero
>>>>
>>>>
>>>> El lun, 23 nov 2020 a las 22:20, BHARATH RAM (<br11 at iitbbs.ac.in>)
>>>> escribió:
>>>>
>>>>> Hello everyone,
>>>>>
>>>>> I am interested in performing resampling of a fine resolution DEM
>>>>> Model (which is imported as a GTiff file in grass GIS). I tried the
>>>>> following 2 approaches and neither of them was successful. Can you help me
>>>>> figure out where I went wrong? Thanks in Advance.
>>>>>
>>>>> *Details about originally imported DEM model:*
>>>>> *projection: 3 (Latitude-Longitude)*
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> *zone: 0datum: wgs84ellipsoid: wgs84north: 29Nsouth:
>>>>> 28:06Nwest: 72Eeast: 77:24Ensres: 0:00:01.08ewres:
>>>>> 0:00:01.08rows: 3000cols: 18000cells: 54000000*
>>>>>
>>>>> *MY AIM:* To convert a fine grid DEM (30 m resolution) to coarse grid
>>>>> DEM (8km)
>>>>>
>>>>> *APPROACH 1: Using r.resample.rst*
>>>>>
>>>>> *g.region raster=test5_r1*
>>>>>
>>>>> *r.resamp.rst input=test5_r1 ew_res=8000 ns_res=8000 *
>>>>> In this second command's execution, I get error (log attached below):
>>>>> * Processing all selected output files will require **4
>>>>> bytes of disk space for temp files.*
>>>>>
>>>>>
>>>>>
>>>>> * Temporarily changing the region to desired resolution
>>>>> ... Changing back to the original region ...
>>>>> Percent complete: ERROR: Unable to seek: Invalid argument*
>>>>>
>>>>> *APPROACH 2: Using r.resample.stats*
>>>>> *g.region raster=test5_r1*
>>>>> *g.region nsres=8000 ewres=8000 *// problematic part
>>>>> *r.resamp.stats -w input=test5_r1 output=test5_r1_8km*
>>>>> In this second command's execution the entire map seems to get
>>>>> clubbed into a single large pixel. I have attached results of *g.region
>>>>> -ap* below:
>>>>> * projection: 3 (Latitude-Longitude)*
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> * zone: 0 datum: wgs84
>>>>> ellipsoid: wgs84 north: 29N
>>>>> south: 28:06N west: 72E east:
>>>>> 77:24E nsres: 0:54 ewres:
>>>>> 5:24 rows: 1 cols: 1
>>>>> cells: 1*
>>>>>
>>>>>
>>>>> Also, can you tell me which method will be more suitable for my final
>>>>> goal?
>>>>>
>>>>> Thanks & Regards,
>>>>> Bharath
>>>>>
>>>>>
>>>>>
>>>>>
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