Zooming to match a raster scale

[Nick Dabner]

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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