Zooming to match a raster scale
dabner at OPTUSNET.COM.AU
Mon Apr 25 19:32:30 EDT 2005
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.
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
> - 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
> 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
> Definately info like this is helpful for us non-geo-versed. Need this
> kind of info in the --verbose docs!
> 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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