| Re: lon-lat coordinates for Geotiff images? [message #72034] |
Wed, 04 August 2010 05:49  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Dave Poreh writes:
> I have some geotiff images (DEM). I can read these files with:
> im=read_tiff(file)
> And I can contour them:
> CONTOUR, im, /FILL, NLEVELS = 7
> But in the contour I get the pixel locations in the x-y axis (local
> coordinates). Does anybody know how could I get lon-lat coordinates in
> x-y axis?
In a contour plot you have some kind of regular (or irregular)
grid. That is to say, your labels are labels of straight lines.
That is seldom true of a map, where straight lines don't seem
to exist.
There is a grid of straight lines associated with a map-
projected image, however. This is the XY grid, which can
be read directly from the GeoTiff file:
http://www.dfanning.com/map_tips/tiffoverlay.html
I would create these vectors using tools from the
Catalyst Library:
mapCoord = GeoCoord('mygeoTiffFile.tif')
mapCoord -> GetProperty, XRANGE=xr, YRANGE=yr
image = Read_Tiff(('mygeoTiffFile.tif')
s = Size(image, /DIMENSIONS)
xvec = Scale_Vector(Findgen(s[0]), xr[0], xr[1])
yvec = Scale_Vector(Findgen(s[1]), yr[0], yr[1])
Then, you could draw your contour plot:
pos = [0.15, 0.15, 0.9, 0.9]
Contour, Reverse(image,2), xvec, yvec, XStyle=1, $
YStyle=1, Position=pos, NLEVELS=7, /FILL
If you wanted to know the latitude and longitude of
any point in the image, you could convert the XY location
to a lon/lat location:
mapStruct = mapCoord->GetMapStructure()
lonlat = Map_Proj_Inverse(x, y, MAP_STRUCTURE=mapStruct)
lon = lonlat[0]
lat = lonlat]1]
If you wanted to see a lat/lon map grid on your contour map, you
could easily add it like this:
grid = Obj_New('Map_Grid', MAPCOORD=mapCoord, /AUTODRAW_GRID, $
COLOR='gray')
mapCoord -> SetProperty, POSITION=pos, GRID_OBJECT=grid
mapCoord -> Draw, /OVERLAYS
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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