[gdal-dev] How to convert a GIF (being a grid with value in channel B from RGB) to XYZ?

[Stefan Keller]

Hi Chaitanya kumar

Many thanks for the hints. I kow VRT from OGR but was not sure if this
works here too.

What I'm still wondering is, how to accomplish the transformation.
It's rather a coord. translation process not comparable to warping:

> The GIF size (= grid size) is 400x400 pixel with a cell size of 1000m.
> The following is known about the geolocation (Switzerland,
> EPSG:21781/CH1903): xmin: 481000, ymin: 38000 and xmax: 881000, ymax:
> 438000.

So I'd like to do some shifting and scaling before XY-coordinates are
output (Z coming from the Colortable).

Yours, S.


2011/7/25 Chaitanya kumar CH <chaitanya.ch at gmail.com>:
> Stefan,
>
> You can convert the integer codes to float values using a VRT file[1] for
> each gif file.
> Create a VRT file with the ColorTable element. Add the SimpleSource to point
> to your gif and the SourceBand element as 3 to point to B band.
>
> Once you create a VRT file for the GIF, you can use it like any other raster
> file. GDAL interprets the elements in it and applies the conversions
> internally. Use it with gdal_translate or gdalwarp (if you want to warp it
> to a new coordinate system) to create an ASCII Grid file.
>
> [1]: http://www.gdal.org/gdal_vrttut.html
>
> On Mon, Jul 25, 2011 at 12:39 AM, Stefan Keller <sfkeller at gmail.com> wrote:
>>
>> I have a raster/grid file transformation and conversion question:
>>
>> The input are hunddrets of GIF files which in fact are a grid about
>> rainfall observations. The files have 8 BitsPerPixel (0..255). The B
>> channel (integer value) from RGB obviously represents the
>> precipitation intensity in [mm/h] according to a lookup table (0=>0,
>> 1=>0.2, 2=>0.32, 3=>0.5, 4=>0.8, 5=1.3, 6=>2, 7=>5, 8=>8 9=>13 10=>20,
>> 11=>32, 12=>50, 13=>50, 14=>80, 15=>130).
>>
>> The GIF size (= grid size) is 400x400 pixel with a cell size of 1000m.
>> The following is known about the geolocation (Switzerland,
>> EPSG:21781/CH1903): xmin: 481000, ymin: 38000 and xmax: 881000, ymax:
>> 438000.
>>
>> The target format is ASCII Gridded (XYZ)? or Arc/Info ASCII Grid
>> (AAIGrid), with e.g. the following on each line: "481000 38000 0.32",
>> meaning the transformed coordinates x and y and the float value (as
>> indicated above in the lookup table).
>>
>> I know OGR but I'm new to GDAL: How would you accomplish this task?
>>
>> Transforming with gdal_transform then converting with gdal_translate ?
>> Or do I have to write something myself (perhaps based on gdal2xyz.py)?
>>
>> -S.
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>> gdal-dev at lists.osgeo.org
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>
>
>
> --
> Best regards,
> Chaitanya kumar CH.
>
> +91-9494447584
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>