Zooming to match a raster scale

[Ed McNierney]

Nick -
 
gdalwarp lets you specify the desired output resolution with the -tr
parameter, so you can say you want the output image to be 2.59 m/p.
 
     - Ed
Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: +1 (978) 251-4242
Fax: +1 (978) 251-1396
ed at topozone.com 


________________________________

From: Nick Dabner [mailto:dabner at optusnet.com.au] 
Sent: Monday, April 25, 2005 7:33 PM
To: Ed McNierney
Cc: MAPSERVER-USERS at lists.umn.edu
Subject: Re: [UMN_MAPSERVER-USERS] Zooming to match a raster scale


Ah, yes. I see. Shame I can't specify I want 2.59 metres per pixel
rather than xpixels and ypixels. This would allow conversion of a folder
of various sizes images to a consistent scale. 

Cheers, 

Nick 

On 25/04/2005, at 11:15 PM, Ed McNierney wrote: 


        Nick - 
        Gdalwarp "warps" an image from one representation to another.
That can be a change in projection, or a change in spatial resolution,
or both (or neither, I suppose <g>).  I wasn't specifically talking
about creating pyramids, although that is another option you should
investigate. 
             - Ed 

        Ed McNierney 
        President and Chief Mapmaker 
        TopoZone.com / Maps a la carte, Inc. 
        73 Princeton Street, Suite 305 
        North Chelmsford, MA  01863 
        ed at topozone.com 
        (978) 251-4242 

        From: UMN MapServer Users List
[mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On Behalf Of Nick Dabner 
        Sent: Monday, April 25, 2005 1:58 AM 
        To: MAPSERVER-USERS at LISTS.UMN.EDU 
        Subject: Re: [UMN_MAPSERVER-USERS] Zooming to match a raster
scale 

        Ed, 

        Thanks for the quick response. Followed your workings and got
what is a near perfect match for another scale. In doing so, I can see
how is resolution is used, which was some explanation I hadn't found
detailed elsewhere. Kept it at 72 dpi. 

        Yes, preview is slow, but the results are nice. Gdalwarp changes
projections if I am not mistaken. Do you mean gdal_translate to create
the raster pyramids? Will have to give this a try later today as my
rasters do vary, especially with inner-city areas. It loooks like I
would supply the output size in pixels (x,y) to get the the desired
scale. 

        Definately info like this is helpful for us non-geo-versed. Need
this kind of info in the --verbose docs! 

        Cheers, 

        Nick 

        On 25/04/2005, at 11:55 AM, Ed McNierney wrote: 


                Nick - 
                I'm not sure what your "unit" is supposed to mean, but I
can describe the process for taking the pixel size and finding the right
MINSCALE/MAXSCALE values. 
                The MINSCALE and MAXSCALE values are the denominators of
scale ratios, like 1:10,000 scale.  MINSCALE 10000 means "a minimum
scale of 1:10,000", where one meter on the map equals 10,000 meters on
the ground.  The problem becomes, "what's one meter on the map"? 
                The RESOLUTION statement answers that question.  The
default value is 72, meaning that 72 pixels in a MapServer output image
are presumed to occupy one inch on the screen.  This is actually
unlikely to be an accurate value unless you're running a 20-year-old
Macintosh, but it often doesn't matter - it's mostly used for internal
calculations. 
                So, if 72 pixels are presumed to cover one inch on the
screen, and your image has a source resolution of 1.26956817 meters per
pixel, then one inch on the screen will show 72 * 1.26956817 meters or
91.40890824 meters of image.  Since 91.40890824 meters equals 3598.7687
inches, this means one inch on the screen shows 3598.7687 inches on the
ground - a scale ratio of 1:3598.7687 
                If you displayed that image at the "scale" of 3598.7687
you should get a pixel-for-pixel replica of your source image.  If you
use a different RESOLUTION statement, you need to adjust accordingly. 
                For calculating the IMGEXT values, you simply need to
make the X extent and Y extent equal the number of (X or Y) pixels in
the image times 1.26956817. 
                Yes, there are better-quality ways of resampling images,
but - as you saw in Preview - they're slow.  MapServer doesn't use them,
but you can use various tools (including gdalwarp, from the GDAL
distribution) to resample images offline and create "pyramids" of
varying resolutions.  This will improve the display quality of your
images while keeping the performance fast - at the expense of extra disk
space and preprocessing work. 
                    - Ed 

                Ed McNierney 

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