[GRASS-user] Unable to perform resampling

Mon Nov 23 14:00:28 PST 2020

[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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