[GRASSLIST:3012] Projection conversion help

H. Fernández ferga1510 at hotmail.com
Mon Jan 28 09:27:47 EST 2002

Hi everyone from Spain:

First of all, you must know that I'm an absolute novice in GIS affaires, 
then sorry for all my mistakes. My english is not really good too, then I 
will try to explain my doubt.

I'm working in a project in order to made a mesh scenery for a Flight 
Simulator (FS2002). We have develop a procedure  to obtain the elevation 
datum from a digital map. This map can generate a greyscale map for an area, 
marking each elevation point with a determinate colour (level of grey). 
Then, we have some tools that can "read" this greyscale map and convert in 
an elevation file that can be interepreted by the simulator. The system work 
really good for small areas, an the results are oustanding. We have a 
horizontal resolution of 50 meters/pixels, and a vertical resolution of 11 m 
/level of grey.

The problem is when we try to aplly the process to large areas (I'm now 
working with and area of aprox 250x100 km). The grey scale map is (I think) 
in UTM/WGS64 coordinates, but our simulator tools expect (I think) a 
Mercator file, a file where horizontal pixels (same file) has the same 
latitude values in degrees, and vertical pixels (same column) has the same 
longitude values in degrees. In our bitmap, if you draw the grid lines, you 
can see that pararell lines are not horizontals, they curve and falls. The 
effect is that dem file read wrong latitude values, and the scenery is 
generated in wrong position, more error when points are far away of NW 
corner. Of course, the same occurs for longitude points.

After a lot of hours reading and searching all over the net, I think GRASS 
(and the function r.proj) is the tool I need. I install in my PC a LINUX, 
download GRASS and run many test without success. r.proj give me an error 
"input map is outside current region", even if I try to define the output 
region larger than original one. I think is because my definition of 
"mercator" projection is not correct.

Then, I ask us for help about how made the definition of output projection 
to obtain a ortoedric conversion of my bitmap file. I read too about 
pictures rectification, but I think this will be not my way.

Here you are the information about my original file:

Asturias3 :  4887x2131 pixels.

Area :       NW :  156650 m (7º 16' 01,419")
                  4855250 m (43º46' 06,070")

             SE :  400750 m (4º 12' 57,653")
                  4748800 m (42º53' 07,247")

Projection ? : UTM - WGS94 - Huso 30

I really apreciate your patience to read this, and any help and support you 
can give me. Thank you very much in advance.

See you

Hector Fernandez
ferga1510 at hotmail.com

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