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I have loaded a DEM of the Topanga quad that covers part of the Santa Monica
Mountains in Los Angeles County. It is a 30m x 30m DEM, in UTM Zone 11
projection, NAD 27, and have created a .img of the DEM.
I used SURFACE to create a slope and aspect model of the quad. I need to get
this file into Atlas GIS, so I had to convert the .img file to a .vec file. I
used the POINTVEC module to do this.
I then used the ATLIDRIS in the export function to create a .bna file that Atlas
GIS can read. The .bna files are created correctly and they import perfectly
into Atlas GIS. I created a 30m x 30m grid cell pattern of polygons using
CELLBNA, a utility program. These cells are EXACTLY 30 meters by 30 meters. No
question about them. The cells are correct. I created a .bna file from the
original DEM using a program written by the late, great Sol Katz. The elevation
points fit PERFECTLY right, dead center in the cells. The original DEM and my
cells are correct.
When I loaded the .bna files of the slope and aspect, they did not line up with
the cells. There is a noticable, progressive shift in the horizontal position
of the points.
Here is the problem.
I would assume that since each cell of the .IMG file represents 30 meters that
the exported points would be 30 meters apart. This is not the case. Vertically
they, the rows of the .img file, are 30 meters apart. An example of the row
coordinates (after export to a .vec file) are:
3764385.5
3764355.5
3764325.5
As you can see, they are 30 meters apart.
However, horizontally, they, the columns, are 30.125 meters apart. An example
of the column coordinates are:
361461.53125
361491.65625
361521.78125
Elsewhere, they are:
350014.21875
350044.34375
350074.46875
Every now and then the horizontal distance from one point (column) to another
point will be 30.09375 meters apart instead of 30.125. Then the cells will go
back to being 30.125 meters apart till there is another shift of only 30.09375
meters as shown below:
x y
350857.71875 3776953.5
350887.34385 3776953.5
350917.9375 3776953.5
350948.0625 3776953.5
The result is that over the horizontal distance of a quad sheet, the extra .125
and .09375 meters per cell, adds up to an additional 48 meters plus of
horizontal distance from one set of points on the west to another set of points
in the east.
I have checked the .vec file before I sent it to the ATLIDRIS module and the
error is in the file BEFORE I used the ATLIDRIS module.
Questions looking for answers:
Has anyone run into this problem before?
Has anyone developed a solution?
Can anyone help me? ;-)
TIA and will SUM.
--
Woody
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"We must cease confusing mastery of software
commands with attaining a grasp of critical
intellectual concepts." -- Duane F. Marble
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