Digital Earth Weblog

The current hype over the imagery available in Google Maps/Earth is driven by the fact that it is high resolution, and less than a few years old. What might surprise users of the Google tools is that there comes a point in time where aerial and satellite imagery starts to increase in value as it gets older. It can be used to show the changes that have occurred over time.

A lot of old imagery is rotting in tape archives and storage rooms, but yesterday I had the rare privilege of working with some aerial photography of Sydney captured in 1943. Yep, forty three. A government agency had the original negatives in their archive. One person made it his mission to not let it disappear and had it all scanned.

Part of the series has now been orthorectified, mosaiced, and compressed into ECW format with a cell size of 0.2 metres. My minor task was to add it to their Image Web Server website. Anyone in the organisation can now browse this imagery alongside the latest aerial photography. Very cool.

More commentary on the ESRI Image Server is available from the Directions Magazine blog and from Dave Bouwman.

I guessed right. Image Server is based on PromptServer.

ESRI have released a dedicated Image Server product. Details are thin at the moment, but it looks like it uses a file-based approach that sounds similar to PromptServer. If I’m reading this right it sounds like the complete opposite of storing raster in ArcSDE. Interesting times.

Via David Maguire, an ESRI blogger.

PromptServer is a geospatial image server that I found out about when reading the GIS Monitor newsletter. The approach to managing imagery that was described in the article was quite interesting so I did some quick web research.

Disclaimer: What follows is based solely on the information available from the www.promptgeo.com website. Your reality may vary. (No, this post is not an April Fools joke.)

Map servers can be divided into two groups:

• Those that render map images on the server (e.g. ArcIMS, MapServer, and many others), and
• Those that do the rendering in the client (e.g. iDelve, Image Web Server).

PromptServer is a server-side renderer, but it differs significantly from other products in that group.

Due to the size of geospatial image files, the typical approach to preparing imagery for serving by a map server is:

1. Obtain all the required image data.
2. Use image processing software to create a physical image product. e.g. A seamless mosaic or tiles that form a mosaic.
3. Archive the original data, and any useful intermediate products.
4. Store the image product in a file system or database, ready for use by the map server.
5. Create the metadata that describes image product. (This should be step no.3, but creating metadata always seems to be done last, if at all.)

Some map servers can do some basic value added processing to that image product before they serve it, like reprojection on-the-fly, but that’s about it. PromptServer is quite different in that you don’t need to create physical image products, and then store and manage those products. It allows you to build up catalogues of georeferenced imagery and create “virtual image products” on-the-fly from those catalogues.

In essence, PromptServer is an image processing engine and map server in one package (that’s my interpretation). Virtual image products are created by defining “processing chains”. Each request to the server invokes a processing chain that creates a map image. (It reminds me of the algorithm approach used by ER Mapper.) PromptServer plug-ins are available for:

• Band processing for multispectral imagery
• Clipping
• Histogram statistics extraction
• Enhancement using look up tables and filters
• Pan sharpening
• Warping
• Orthorectification
• and more…

There are several data management benefits to using the virtual image process:

1. You only need to store the original data. No intermediate or final data products physically exist. Less data means easier data management.
2. As more data becomes available should be easy to add it to the appropriate catalogue and make it available to the applications that use it.

Image processing is a CPU, memory and disk intensive activity. PromptServer’s response times are going to depend on the type of image data, the processing that needs to be applied to it and the server hardware is running on. I’d be interested to know what sort of hardware would be required to generate typical image within a reasonable period (less than 5 seconds). PromptServer is certainly positioned well to take advantage of the ever reducing cost of CPU cycles, RAM and disk space.

Footnote: I have sent an email to the good folks at PromptGeo to let them know about this post, and offering them the opportunity to provide feedback or leave a comment. If I’ve got anything wrong I’ll change the post.

If you are looking for aerial photography or satellite imagery, and just want to view it on screen rather than buy it, then www.earthetc.com would be worth a visit. [Disclosure: My company used to sell the software that runs that site.]

If you want to see a truly amazing image have a look at the satellite mosaic of the world. It is 2 terabytes in size, and you can view it in your browser across the Internet. All you need to do is install a small browser plug-in. It works by only downloading the data you need to display your current on screen view. You do not need to download the whole image, that’s the point of the technology.

Here’s how you do it:

1. Visit www.earthetc.com.
2. In the "Location/Type" column of the Imagery table, click on "World".
3. Click on "World TM742" to view the image.

Note: If you haven’t installed the ECW viewer plug-in before you will be prompted to do so.

Now you can pan and zoom as you please. I highly recommend zooming in over the Sahara desert and looking at the sand dunes. Some of the dunes are 1.5km from peak to peak! The geology around Wilpena Pound in South Australia is also worth a look.

If you are wondering why the image has some rather strange colours in it, that is because it has been created using bands 7, 4 and 2 of from the Landsat sensor. So what you are seeing is a visible representation of infrared light reflected from the surface of the Earth. Normally you cannot see infrared light, but this technique is used to highlight interesting geological structures on the Earth’s surface.

There are also natural colour images available at www.earthetc.com. Feel free to explore.

I was looking through the Digital Earth website statistics yesterday and noticed that there are quite a few people visiting the site after searching for aerial photography or satellite images of Sydney. Digital Earth doesn’t sell image data (sorry), we just help clients make it available through their web mapping applications.

If you are one of the people who are searching for terms like:

• aerial maps sydney,
• aerial photo sydney,
• aerial satellite maps,
• images from space of sydney,
• space photos of sydney,
• sydney aerial photos,
• sydney from space, or
• sydney space

then I suggest that you contact the following organisations. Please keep in mind that high resolution aerial and satellite image data costs a lot of money to capture and process, and is priced accordingly.

• AAM Hatch – aerial and satellite.
• Lands NSW – aerial.
• Sinclair Knight Merz – aerial and satellite.
• WebMap – aerial.

If you are looking for oblique (scenic) aerial photography have a look at:

• AirView


• SkyShots Aerial Photography


• Jumbo Aerial Photography

I’m planning to add a directory of aerial and satellite data providers that cover other parts of Australia, so if there are any vendors out there who would like to be listed please leave a comment or send an email [anti-spam device].

[Update, 2004-11-30] If you are interested in viewing aerial or satellite imagery online, rather than purchasing it, I’ve written a post that explains how to view some interesting data for free.

[Update, 2005-04-05] Added SkyShots Aerial Photography and Jumbo Aerial Photography.