Comments on: Google, Microsoft, and the USGS http://jimthompson.org/wp/2005/04/08/google-microsoft-and-the-usgs/ A clean, well-lighted blog. Sat, 16 May 2009 13:22:18 -0400 http://wordpress.org/?v=2.8.4 hourly 1 By: Jim http://jimthompson.org/wp/2005/04/08/google-microsoft-and-the-usgs/comment-page-1/#comment-32 Jim Sun, 05 Jun 2005 18:31:23 +0000 /?p=73#comment-32 Thanks for the notes, Tim. I was aware that there was a second set of imagery in addition to the USGS Urban Areas imagery, but I didn't know what it was called. And yeah, you're right that Google/Keyhole probably isn't doing their own orthorectification. Why would they, if the USGS is already doing it? But it never occurred to me that the difference might be simply due to the map projection. Thanks for the notes, Tim.

I was aware that there was a second set of imagery in addition to the USGS Urban Areas imagery, but I didn’t know what it was called.

And yeah, you’re right that Google/Keyhole probably isn’t doing their own orthorectification. Why would they, if the USGS is already doing it? But it never occurred to me that the difference might be simply due to the map projection.

]]> By: Tim Peterson http://jimthompson.org/wp/2005/04/08/google-microsoft-and-the-usgs/comment-page-1/#comment-31 Tim Peterson Sat, 04 Jun 2005 01:30:52 +0000 /?p=73#comment-31 A few notes: TerraServer does indeed serve only USGS images, but there are two datasets available (as I'm sure you know). The first is the NAPP (National Aerial Photography Program) 1 meter per pixel grayscale imagery, which covers nearly all of the US. (Some of these images were actually taken in infrared, but they've been made grayscale for TerraServer.) The second is the USGS Urban Areas 1 foot resolution color imagery, which covers 30+ major cities. Some sites (such as GlobeXplorer) fall back on the NAPP imagery when they don't have anything better to display. However, Google (and hence Keyhole) apparently accepts only color imagery, so they instead default to the lower resolution Landsat for the unimaged reaches of the country. Keyhole does make use of the Urban Areas imagery, which you're seeing in most of the images above. I rather doubt that Keyhole does their own orthorectification, and even if they did it wouldn't come out as squished as you see on Google compared with TerraServer's rendition. The difference between the two is actually the map projection being used. TerraServer uses the UTM (Universal Transverse Mercator) projection, which slices the globe up into zones 6 degrees of longitude wide and then flattens them such that the resulting image pixels closely approximate square meters on the ground. This makes it easy to compare distances and directions on the image, but it also creates gaps between UTM zones. As a compromise between accuracy, user-friendliness, and easy programmability, Google Maps uses a simple lat-long projection, where pixels on the X and Y axes correspond directly to fractions of degrees of longitude and latitude, respectively. Due to the Earth's curvature, however, this results in a horizontal stretching of the image in areas far from the equator. This is why the Google "Satellite" image is compressed vertically relative to TerraServer even though they both use the same source image. (The Maps view actually has a constant scale factor built in, which returns the map to approximately correct proportions for viewing the US. This scaling wasn't applied to the Satellite imagery, though, perhaps because Keyhole already had everything in normal lat-long projection.) As you noted, the USGS Urban Areas imagery has 0.3m (1ft) resolution, which on TerraServer is scaled to 0.25m resolution (to match the powers-of-two zoom levels). Google's maximum zoom level has a resolution of 1.2m/pixel vertically, and a horizontal resolution that ranges from 0.8m to 1m as you move from north to south in the US (due to the projection). So, Google's zoom levels are approximately equivalent to TerraServer's, all the way down to 1m. --Tim A few notes:

TerraServer does indeed serve only USGS images, but there are two datasets available (as I’m sure you know). The first is the NAPP (National Aerial Photography Program) 1 meter per pixel grayscale imagery, which covers nearly all of the US. (Some of these images were actually taken in infrared, but they’ve been made grayscale for TerraServer.) The second is the USGS Urban Areas 1 foot resolution color imagery, which covers 30+ major cities. Some sites (such as GlobeXplorer) fall back on the NAPP imagery when they don’t have anything better to display. However, Google (and hence Keyhole) apparently accepts only color imagery, so they instead default to the lower resolution Landsat for the unimaged reaches of the country. Keyhole does make use of the Urban Areas imagery, which you’re seeing in most of the images above.

I rather doubt that Keyhole does their own orthorectification, and even if they did it wouldn’t come out as squished as you see on Google compared with TerraServer’s rendition. The difference between the two is actually the map projection being used. TerraServer uses the UTM (Universal Transverse Mercator) projection, which slices the globe up into zones 6 degrees of longitude wide and then flattens them such that the resulting image pixels closely approximate square meters on the ground. This makes it easy to compare distances and directions on the image, but it also creates gaps between UTM zones. As a compromise between accuracy, user-friendliness, and easy programmability, Google Maps uses a simple lat-long projection, where pixels on the X and Y axes correspond directly to fractions of degrees of longitude and latitude, respectively. Due to the Earth’s curvature, however, this results in a horizontal stretching of the image in areas far from the equator. This is why the Google “Satellite” image is compressed vertically relative to TerraServer even though they both use the same source image. (The Maps view actually has a constant scale factor built in, which returns the map to approximately correct proportions for viewing the US. This scaling wasn’t applied to the Satellite imagery, though, perhaps because Keyhole already had everything in normal lat-long projection.)

As you noted, the USGS Urban Areas imagery has 0.3m (1ft) resolution, which on TerraServer is scaled to 0.25m resolution (to match the powers-of-two zoom levels). Google’s maximum zoom level has a resolution of 1.2m/pixel vertically, and a horizontal resolution that ranges from 0.8m to 1m as you move from north to south in the US (due to the projection). So, Google’s zoom levels are approximately equivalent to TerraServer’s, all the way down to 1m.

–Tim

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