Horshamite (12/10/2007)
This is an excellent piece of work. Sorry to demonstrate my ignorance but what determines the boundary of the high-res image? I have never seen such good aerial images of Horsham.
Basically the size of my wallet!
No seriously it is really down to cash. Just to explain:
Virtual Horsham is based on Microsoft Virtual Earth. The basic images used on Virtual Earth are a set of aerial photographs that were taken around 1999 by Getmapping plc for the 'Millennium Map'.
Anybody can freely build a web application based on this platform. We originally took this approach for Visit Horsham and Virtual Horsham but it soon became apparent that some areas for the town centre appeared very out of date on the map. As an example go to Visit Horsham and uncheck the "Show High Resolution Images" tick box. If you do this you will notice that the whole of the forum is missing (as is the Pavilions in the Park).
I started to think about possible ways to integrate new photographs into this application and discussed it with other Virtual Earth developers. One option was to commission custom aerial photography for the affected areas but for various reasons this was not really practicable or cost effective.
I then discovered that Getmapping PLC already had some new high-resolution pictures of the Horsham Town Centre area. After some negotiation with them they agreed to license the use of the images for display on the Visit Horsham and Virtual Horsham websites. That was only the start of the problem!
After chatting with other Virtual Earth developers I found that nobody else (worldwide) had replaced Virtual Earth images on this scale before. Fortunately Microsoft make available a free product to developers that will 'slice up' the source image into the many, much smaller images that are needed for Virtual Earth application. Microsoft themselves gave me some direct assistance with this, as did some other Virtual Earth developers. I am very grateful to these people because the result is that we are the first town in the UK to do this and quite possibly the first in the world on this scale. This site has become a model for others looking to do the same and this month I managed to get it mentioned in the prize letter in .net magazine (a national publication sold in W H Smiths etc.)
Anyway back to the cost. There is an initial (size dependant) licensing fee for the new image that has been used. This image has since been sliced-up into about 2GB of smaller images. These smaller images are the ones that you see as you move around and zoom in on the map. The web server has to process these and if multiple users are concurrently online this processing represents a considerable load on the server. If you know a little maths you will appreciate that data stored in these images will increase proportionally to the width times the height of the source image. In other words if you double the width and the height you need four times the data to store it. If we keep increasing the area for the source image we will soon hit a point when a single server can no longer handle the processing. We could overcome this by increasing the server power but it would obviously involve considerably more cost.
Virtual Horsham is effectively being funded by the revenue from the Visit Horsham subscribers so it initially made sense to choose an area that covered the distribution of those subscribing businesses. Currently this just about cost effective.
It would not currently be cost effective to to expand the area covered by the high-resolution images but hopefully in the future we will be able to . It is also likely that at some stage Microsoft will update the underlying images but, if they do, their new images may still lag the latest images that will be available from Getmapping.
Thanks for you interest. I hope this explains things for you.