Hi Brent!
Thank you so much for your effort, I definitely owe you a drink!
This is a really cool overview of world files and mapping stuff at all!
Greetings,
Pascal
Brent Fraser wrote:
Pascal,
I would guess the data is in a geographic coordinate
system. This means the units of coordinates and pixel sizes
for the image would be degrees not meters. I would also
assume that the image covers the entire Earth, not just a
part of it, so:
Calculating the X (longitude) pixel size, 360 degrees covers
the Earth all the way around:
360 degrees / 8192 pixels = 0.0439453125 degrees per pixel
in X
Calculating the Y (latitude) pixel size, 180 degrees covers
the Earth pole to pole:
180 degrees / 4096 pixels = 0.0439453125 degrees per pixel
in Y
Notice the pixel size in X is the same as Y. This is
typical for a raster map (but not a hard rule).
---------------
And now for a reality check. Approximately how many
meters-on-the-ground is each pixel?
0.0439453125 degrees/pixel * 60 minutes/degree * 60
seconds/minute * 30 meters/second (approx) = 4700
meters/pixel
Interesting information, but not needed for the tfw file.
---------------
So back to the calculations for the tfw file:
The coordinates of the upper-left corner of the upper left
pixel would be
X = -180 degrees
Y = 90 degrees
But the tfw file specifies the MIDDLE of the upper left
pixel (so we need to subtract half a pixel width from each
coordinate):
X = -180 + 0.0439453125/2 = -179.97802734375
Y = 90 - 0.0439453125/2 = 89.97802734375
So the tfw file would be:
0.0439453125
0
0
-0.0439453125
-179.97802734375
89.97802734375
One more step for Mapserver:
Since the tiff file is not a geotiff, you need to tell
mapserver the coordinate system of the raster file. So your
layer definition in the the map file would be (note the
PROJECTION object):
########## NASA BLUE MARBLE ###########
LAYER
NAME "satellite"
DATA "/var/www/odamap/blue_marble.tif"
TYPE RASTER
PROJECTION
"geographic"
END
METADATA
"wms_title" "ODAdata WMS Server SATELLITE"
END
END
#######################################
