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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | RE: GISList: Scale conversion formula or chart |
| Date: |
11/15/2001 04:58:45 PM |
| From: |
Graeme Brooke |
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Jason,
There is a chart at http://www.grime.net/virtuozo/pvalues.html that may be
of use.
A 30 meter DEM means that there is a recorded value for height on a regular
grid where the grid spacing is 30m. This doesn't really equate to "pixels"
as that term (a contraction of "picture elements") usually refers to a
raster image such as a photograph (e.g. TIFF or JPEG).
I must admit that the concept of scale as it relates to digital imagery is a
bit vague as you can very simply change the resolution of an image in
something like Photoshop to get a new scale but this may inadvertently
change the accuracy of the image, so the scale no longer is valid.
Graeme Brooke
-----Original Message-----
From: Jason Lampitt [mailto:jlampitt@yahoo.com\
Sent: Friday, 16 November 2001 2:07 AM
To: gislist@geocomm.com
Subject: GISList: Scale conversion formula or chart
I'm fairly new to the world of GIS so please bear with
me if I get my vocabulary a bit mixed up.
Is there some conversion formula or chart that enables
someone to see how a file such as a 30-meter DEM file
translates into scale (1:24,000)?
When I seen something like "30-meter DEM" does that
mean 30 meters per pixel?
I use a GPS application that has a scale bar which
expresses the scale in standard US terms (200 ft, .25
mi, etc) which corresponds to so many meters per
pixel. So, as an example, 200 ft on the scale bar
represents 2 meters/pixel, 250 ft = 4 meters/pixel,
etc.
I export images from this application into Adobe
Photoshop for further enhancement and I'm trying to
get my mind wrapped around building an accurate scale
for the map and how I can combine different GIS
resource files accurately.
Thanks in advance.
Jason L.
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