[gdal-dev] Having difficulties using gdalwarp

[Simon Knapp]

Hi Frank,

Thanks for the amazingly quick response!

You can access the files at:
https://www.dropbox.com/sh/f326gvclu5f1qjm/GNvp2mZLbU

I was waiting to send this response until the files had finished
uploading (280Mb in total), but I have to pop out for a few hours and
they have not yet finished. If you are going to look at them I would
leave it for a half hour or so (based on the time it has taken so far)
to give them a chance to finish.

The only difference in structure is that the directory referred to as
'clum' is actually 'clumfill_0310. The batch file "warp.bat" contains
the calls I use.

I have also put the input and output histograms (from QGIS) in that
folder (input_hist.bmp and output_hist.bmp) for your convenience.

Thanks again,
Simon

On Thu, Oct 4, 2012 at 10:12 AM, Frank Warmerdam <warmerdam at pobox.com> wrote:
> On Wed, Oct 3, 2012 at 5:06 PM, Simon Knapp <sleepingwell at gmail.com> wrote:
>> Hi List,
>>
>> This is my first post here so please let me know if my question would
>> be better placed elsewhere.
>
> Simon,
>
> This is a good place for the question.
>
>> I am struggling with re-projecting integer
>> grids using gdalwarp and would appreciate some help.
>>
>> I have two grids containing land use data at different scales:
>>
>> grid 1) a 50m (esri binary) grid in albers,
>> grid 2) a 1km (ascii) grid in geographics.
>>
>> I would like to project these to a common coordinate system with
>> resolution corresponding to grid 1 so I can perform cross tabulations
>> and alike. I have tried going in both directions
>>
>> The calls I have tried are:
>>
>>
>>
>> call 1) re-project and re-sample grid 2 to the same projection as grid
>> 1. I have obtained the parameters from the property pages of grid 1 in
>> QGIS:
>>
>> gdalwarp ^
>> -t_srs "+proj=aea +lat_1=-18 +lat_2=-36 +lat_0=0 +lon_0=132 +x_0=0
>> +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs" ^
>> -te -1888000 -4847000 2122000 -1010000 ^
>> -tr 50 -50 ^
>> -dstnodata -32768 ^
>> nlum.asc nlum_warped.tif
>>
>>
>> call 2) re-project grid 1 to have the same projection as grid 2. Here
>> I have not specified the resolution and was planning to also re-sample
>> grid 2 to the resolution of the result. I get the parameters from the
>> QGIS property pages of grid 2:
>>
>> gdalwarp ^
>> -t_srs "+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +no_defs" ^
>> -te 112.505 -44.0055 154.005 -9.995 ^
>> -dstnodata -32768 ^
>> clumfill clum_warped.tif
>>
>>
>>
>> In the first call I get 24Gb of complete rubbish. When I open the
>> result in QGIS noting is shown (visually) and the cell values contain
>> what appears to be uninitialised junk.
>>
>> In the second call, I get a grid that looks similar to what I want,
>> but the cell value ranges are different ([100, 663] in the input and
>> [0, 663] in the result) and the histograms are different. I thought
>> that by (implicitly) choosing nearest neighbour the values in the
>> output must be a subset of those in the input.
>>
>> Can anyone suggest what might be going wrong?
>
> In the second case, I imagine the zeros represent
> nodata pixels - that is pixels for which there was no source data
> due to the change of coordinate system.
>
> It is hard to know what went wrong in the first case.  Is
> the source file small enough that you could make it available
> for me to test with?
>
> Best regards,
> --
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Software Developer