[Geotiff] Relationship Between Model Tie Point and Raster Space

[David Burken]

Clint,

Elevation is usually interpreted as "PixelIsPoint". There is usually an 
overlap pixel; hence 1201, 3601. So for instance in dted and srtm with a 
one degree cell with 1 arc post spacing you have a post on both 
longitude and latitude lines. If it were area, it would edge to edge and 
you would only have 3600 post for 1 arc second instead of 3601.

Hope that helps,
Dave


Tillerson, Clint wrote:
>
> Hello,
>
> I’m trying to understand the relationship between the model tie point 
> (upper left corner of the geographic domain) and the raster space 
> (PixelIsPoint vs. PixelIsArea) to ensure I am properly assigning 
> geographic coordinates to the data points in the file. I am extracting 
> elevation data (NED or SRTM) for preprocessing for another program.
>
> If someone could verify my interpretation is correct…
>
> Suppose:
>
> * model tie point = (0, 0, 0, -112.0, 39.0, 0),
>
> * raster type = PixelIsArea
>
> * pixel scale = 1 arc-second in X and Y directions 
> (0.000277777777777778, 0.000277777777777778, 0)
>
> I interpret this to mean the upper left corner of the first pixel 
> (0,0) is located at lon = -112.0 and lat = 39.0. However, the 
> elevation for that first pixel, bound by points (0,0) and (1,1) is 
> represented at the center of the pixel derived by adding 1/2 
> arc-second to the longitude and subtracting 1/2 arc-second to the 
> latitude (i.e., adjust lat/lon to center of pixel).
>
> Similarly, if the raster values for the model tie point were 0.5, 0.5, 
> 0, this would mean that lat/lon supplied in the model tie point 
> already represents the center of the pixel and are the coordinates for 
> the first elevation.
>
> In the original example above, if the raster type were PixelIsPoint, 
> then the first elevation is represented by the coordinates specified 
> in the model tie point (i.e, do not adjust the coordinates by 1/2 
> pixel). If the raster values were 0.5, 0.5, I would need to derive the 
> coordinates for the first elevation by subtracting 1/2 arc-second from 
> the longitude and adding 1/2 arc-second to the latitude (i.e, adjust 
> geographic coordinates to raster point (0, 0).
>
> Could someone verify my interpretation correct or show me the error of 
> my ways?
>
> Thanks so much
>
> Clint
>
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