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Julian -
No, that's not correct. As Greg pointed out, MINSCALE
and MAXSCALE are scale ratios (actually, scale ratio denominators) and as such
have no units. Therefore, changing your mapfile UNITS will have no
effect.
A map scale must refer to two measurements - the size of
the object on the map, and the size of the real, physical object it
represents. I'm not sure what your "3280m zoom" means, since you're only
providing one of those two numbers - you must mean that 3280m on the ground is
displayed as some particular size on the map, but you don't say what that size
is.
There is a problem with digital images and scale
ratios. If MapServer creates a digital output image, it can't really tell
how large it is on your screen or printer. You can fiddle with the
horizontal and vertical size controls on your monitor and change the size of
that image and MapServer can't possibly know that. So if MapServer refers
to a scale of 1:10,000, it means that (for example) 10,000 centimeters on the
ground (100 meters) equals one centimeter on the screen. But you don't
know how many pixels per centimeter your monitor is showing, so you can't really
know how much of your image is displayed in that one centimeter on the
screen.
To get around this problem, MapServer assumes by default
that your output device has a resolution of 72 pixels per inch. The
RESOLUTION statement in your mapfile can be used to change this default to a
different value. But it's still just an estimate, since you don't know the
real value on your user's screen. Setting the RESOLUTION simply allows you
do to the math differently for MAXSCALE and MINSCALE calculations, which you may
find more convenient (it also affects a few other things).
The RESOLUTION and MAX/MINSCALE values together can be used
to make some sense out of this. In my example above, we now know that
MapServer thinks that there are 72 pixels in one inch on the screen. A
scale of 1:10,000 means 100m on the ground = 1cm on the screen. There are
2.54cm in one inch, so 1cm = 28.34 pixels (at 72 pixels per inch). Your
image shows 100m in 28.34 pixels, so each pixel represents 3.53 meters on the
ground. That value doesn't change if you modify the size of the image, and
is a more sensible way to think of digital image scale (meters per
pixel).
So - a MAXSCALE value of 12000 means that the maximum scale
ratio is 1:12,000. At 72 pixels per inch, that means that 72 pixels (one
inch) = 12,000 inches on the ground. 12,000 / 72 = 166.667 inches per
pixel, so each pixel is 4.2344 meters on the ground. When a pixel would
cover more than 4.2344 meters of ground, that layer will be turned off because
you exceeded the MAXSCALE setting.
I can't figure out your "scale factor" without the other
half of your scale definition (3280m = ???), but I'd be happy to work through
that example if you provide the additional information.
- Ed
Ed McNierney From: UMN MapServer Users List [mailto:[EMAIL PROTECTED] On Behalf Of Julian Parker Sent: Tuesday, August 29, 2006 12:45 AM To: [email protected] Subject: [UMN_MAPSERVER-USERS] FW: MINSCALE & MAXSCALE Units Greetings
list, It was my assumption that I could
set UNITS as meters in a layer and that the MIN & MAX SCALE values would
utilize these units. This does not seem to be the case – a MAXSCALE value of
12000 on one layer will have the layer display at 3936m zoom. Another layer with
a MAXSCALE of 10000 will display at 3280m zoom. The ‘scale factor’ equals about
3.05. How is it that this ‘scale factor’ can be altered or the units used for
scale changed? What am I missing? Thanks in
advance! Regards Digital Mapping
Solutions MapInfo
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