Elevation is usually interpreted as "PixelIsPoint". There is usually an

overlap pixel; hence 1201, 3601. So for instance in dted and srtm with a

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.

>

> 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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