Thanks for a very nice reference. Bookmarked.

rmfr

However, line 4 still seems to be twice what it should.
7143.334479058464
0.00000
0.00000
-14272.22052182727
-25716004.124610472
25689996.939289085

Shouldn’t it be: -7136.110260913635?

BTW: the 7201pxW by 3601pxH is Equirectangular Projection. Thus, the height covers only 180 degrees, but the width will cover 360 degrees. Does the JS script assume height also covers 360 degrees?

Thanks for the prompt reply.

rmfr

7143.334479058464
0.00000
0.00000
-14272.22052182727
-25716004.124610472
25689996.939289086

The fourth value, delta-y, still doesn’t agree. Are you sure of your input and expected output? E.g., is the image height correct?

It’s a simple-minded bit of code that works for what I needed. The code can be viewed at http://egb13.net/jscripts/worldfile.js. Any suggestions for improvements are appreciated. One thing that would come in handy sometimes would be to let you enter the pixel size, and have it give you the resulting image dimensions, for when I want to crop a map image to a specific extent, e.g.

I get these results:
0
0.00000
0.00000
0
25719575.79185
25697133.04955

However, should the results actually have been:
7143.334479
0.00000
0.00000
-7136.110261
-25716004.12461
25693564.99442

Just wondering…

rmfr

http://maps.google.com/?ie=UTF8&ll=36.779042,-103.971863&spn=0.370679,0.801315&z=11

The “ll=” parameter is the latitude and longitude of the center of the map. The “spn=” parameter must stand for “span”, because it appears to be the height and width of the displayed map in degrees. Test this by trying to find the coordinates of the upper-left corner: add half the vertical span to the latitude and subtract half the horizontal span from the longitude. (Moving northwest increases latitude and decreases longitude.) Applying that formula to the URL above gives this new URL:

http://maps.google.com/?ie=UTF8&ll=36.964381,-104.372520&spn=0.370679,0.801315&z=11

That indeed looks like the upper-left corner of the original map, and what was at the center (an extinct cinder-cone volcano that you can drive to the top of and hike through the crater) is now at the lower-right corner.

Since you’re already given the degree span, you don’t really need this calculator; divide the vertical and horizontal span values given in the link URL by the pixel dimensions of the image and you have the 1st and 4th parameters for the worldfile.

But I don’t know what projection Google uses to display their maps. Is it simple latitude x longitude (EPSG:4326)? Or is it a UTM projection? Or does it vary by the size of the area covered? UTM is not good for maps that cover more than a certain distance horizontally. EPSG:4326 is not good for lots of things. That’s what is used in the video in my Road Trip! posting, and even though that doesn’t cover a terribly large area (a little over 100 mi horizontally) and the counties shown are noticeably flatter than they should be. And a worldfile (at least with the 2nd & 3rd parameters zero) is not good for “curved” projections.

Might this be the wrong approach, though? If you haven’t already investigated Google’s Maps API you might want to. I see lots of places around the internet making pretty good use of it. It’s been a very long time since I dabbled in it and am unable to offer any help with that, not to mention that I don’t know what exactly your after anyway.