[GRASS-user] Unable to perform resampling
BHARATH RAM
br11 at iitbbs.ac.in
Mon Nov 23 14:22:18 PST 2020
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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