Comments on: World file calculator http://egb13.net/2009/03/worldfile-calculator/ Time-lapse, weather, maps, and more Sat, 19 Jun 2010 02:29:07 -0400 http://wordpress.org/?v=2.8.6 hourly 1 By: egb13 http://egb13.net/2009/03/worldfile-calculator/comment-page-1/#comment-56 egb13 Thu, 15 Apr 2010 15:28:42 +0000 http://egb13.net/?p=59#comment-56 Is this program available for download? The JavaScript code for it can be found at http://egb13.net/jscripts/worldfile.js. It's just a couple lines of arithmetic. Is this program available for download? The JavaScript code for it can be found at http://egb13.net/jscripts/worldfile.js. It’s just a couple lines of arithmetic.

]]> By: Donald http://egb13.net/2009/03/worldfile-calculator/comment-page-1/#comment-52 Donald Fri, 26 Mar 2010 15:14:59 +0000 http://egb13.net/?p=59#comment-52 Is this program available for download? Is this program available for download?

]]> By: egb13 http://egb13.net/2009/03/worldfile-calculator/comment-page-1/#comment-23 egb13 Thu, 26 Nov 2009 19:48:47 +0000 http://egb13.net/?p=59#comment-23 Replying to myself... my response to NickMcW is for Google Maps, but he asked about Google Earth. It's a more interesting question, but not one I have an answer to. Yet. Replying to myself… my response to NickMcW is for Google Maps, but he asked about Google Earth. It’s a more interesting question, but not one I have an answer to. Yet.

]]> By: egb13 http://egb13.net/2009/03/worldfile-calculator/comment-page-1/#comment-22 egb13 Thu, 26 Nov 2009 18:41:02 +0000 http://egb13.net/?p=59#comment-22 If in Google Maps you click on "Link" you get a choice two strings to copy and paste. The first one, for email or IM, is the bare URL of the displayed map; if you past it in the browser's address box, you'll get the map you see displayed. That Maps URL looks like this: 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. If in Google Maps you click on “Link” you get a choice two strings to copy and paste. The first one, for email or IM, is the bare URL of the displayed map; if you past it in the browser’s address box, you’ll get the map you see displayed. That Maps URL looks like this:

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.

]]> By: NickMcW http://egb13.net/2009/03/worldfile-calculator/comment-page-1/#comment-21 NickMcW Thu, 26 Nov 2009 16:53:47 +0000 http://egb13.net/?p=59#comment-21 Hi - nice utility, thanks for making it available. It prompts a related question: does anyone know a way to programmatically obtain the corner coordinates of the current view extent in Google Earth? The reason for asking is in order automate the creation of world files - as done with this calculator - for google earth screen-saved images. (The image size, in pixels, will depend on the resolution option chosen in GE; the options available depend on whether the free or pro version is used.) Hi – nice utility, thanks for making it available. It prompts a related question: does anyone know a way to programmatically obtain the corner coordinates of the current view extent in Google Earth? The reason for asking is in order automate the creation of world files – as done with this calculator – for google earth screen-saved images. (The image size, in pixels, will depend on the resolution option chosen in GE; the options available depend on whether the free or pro version is used.)

]]> By: egb13 http://egb13.net/2009/03/worldfile-calculator/comment-page-1/#comment-14 egb13 Tue, 21 Apr 2009 22:42:26 +0000 http://egb13.net/?p=59#comment-14 I've made a minor modification to the code to correctly calculate the location of the center of the corner pixel. It's really trivial -- just adjust by 1/2 of the pixel size in geo coordinates. I’ve made a minor modification to the code to correctly calculate the location of the center of the corner pixel. It’s really trivial — just adjust by 1/2 of the pixel size in geo coordinates.

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