Relationship Between Model Tie Point and Raster Space

classic Classic list List threaded Threaded
2 messages Options
Reply | Threaded
Open this post in threaded view
|

Relationship Between Model Tie Point and Raster Space

Tillerson, Clint
Relationship Between Model Tie Point and Raster Space

Hello,

Im 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

 


_______________________________________________
Geotiff mailing list
[hidden email]
http://lists.maptools.org/mailman/listinfo/geotiff
Reply | Threaded
Open this post in threaded view
|

Re: 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
>
> ------------------------------------------------------------------------
>
> _______________________________________________
> Geotiff mailing list
> [hidden email]
> http://lists.maptools.org/mailman/listinfo/geotiff
_______________________________________________
Geotiff mailing list
[hidden email]
http://lists.maptools.org/mailman/listinfo/geotiff