Posts Tagged ‘Google Maps’

Image Tile Caching, rehashed

Friday, October 2nd, 2009

A little more than a year ago, I wrote about my rather simplistic approach to caching image tiles for the interactive map viewer on PhilaGeoHistory.org. Since that time, I’ve made a few updates that are worthy of a reposting, particularly if you want to use the code for anything.

First, one of our partners, the Free Library of Philadelphia, wanted to use the script, but wanted it in ColfFusion. So I migrated it from PHP to ColdFusion. The two versions work very similarly, so you can see the current version in PHP or ColdFusion.

Second, here are the new features:

  • Store the cached images for different layers in different directories. Seems simple enough. I hesitated to do this previously because you could request multiple layers at once through the WMS script, but in practice I’ve never done it.
  • In ColdFusion, open the resulting image to make sure it is a valid image. Sometimes the file that is returned is garbage, or just an error, and you don’t want to cache it. Also, if the PNG file is smaller than 1K, it’s most likely not an image at all.
  • If the first attempt to retrieve an original tile fails, or results in an error, retry the http request again.
  • Use an appropriate file extension denoting the format of the image. Previously, just the hash of the request string was used as the filename. Now it’s the has plus the correct extension. Makes it possible to set the correct MIME type when serving up a cached image, and makes viewing problematic images in the file system a lot easier.
  • Return an HTTP header telling the client to cache the image. The results of dynamic scripts in general are not cached by browsers, but in this case, significant speed benefits arise from holding onto the images, which are very unlikely to change in any short period of time.

Further features that would be useful include recognizing the geographic extent of each layer and not caching tiles outside of that extend; recognizing empty tiles; and more efficiently storing the resulting cached files.

I don’t expect to invest too much in this, especially since there are much larger systems to do things like this (TileCache is one example). Currently, it’s too complicated for me to consider implementing, but if I need more features, or the project scales larger, I’ll have to migrate to something like it.

Displaying WMS Layers in Google Maps

Wednesday, June 18th, 2008

Image Web Server from ER Mapper offers a WMS reflector that lets you request pieces of JPEG 2000 and ECW images. The trick was to get that WMS data into a Google Maps interface.

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Image Tile Caching

Wednesday, June 18th, 2008

Update (Oct 2, 2009): I’ve written a new post with links to newer versions of this script. Make sure you use the code there, not here.

For the map mosaic viewer I’m working on, the images are generated by ER Mapper’s Image Web Server from the source JPEG 2000 and ECW files. It works great, but can be a little slow, as it has to load very large images (equivalent of 100GB uncompressed) and pull tiles out. Because the viewer always uses the same tiles at each zoom level (defined by a lat/lon bounding box), it made sense to cache the images.

A real simple PHP file seemed to do the trick:

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