How current is the Ordnance Survey map data?
The Ordnance Survey are continually surveying Great Britain and making changes to their map datasets.
They don’t give us an absolute timescale on data deliveries, but it’s approximately quarterly for 1:25,000 Explorer and 1:50,000 Landranger data releases.
Arrival of New Data at Anquet
The first we often knew about it was a package of DVD’s arriving in the post. Thankfully the OS have now implemented a new data download process for their partners, and so we now get an email which notifies us we have updated data to collect.
What has Changed?
The OS don’t tell us! For Standard Definition maps they just supply the map tiles to us that have changed since the last update. For High Definition they send us the whole set of maps each time. I’ve not fathomed out the difference in the logic between data sets!
With the High Definition, we compare the tiles to the last delivery and we can then easily extract the tiles that have changed.
OS 1:50,000 Landranger source map tiles are 20km by 20km, and OS 1:25,000 Explorer source map tiles are 10km * 10km. We can therefore quite easily determine which tiles have changed, but what we can’t ascertain is what has changed in a certain tile short of comparing by eye two map tiles. So the change in one map tile could be a spelling mistake, or a new bridge!
A worse alternative for not knowing what has changed is if the OS as they did quite a few years ago, decide to change some map colours slightly – suddenly the whole data set looks new.
Below are two images.
LEFT: - A complete set of Ordnance Survey 1:50,000 HD map tiles
RIGHT: - the 1st December 2016 update.
The images aren't quite lined up properly, but you can see that for instance a large part of Northern England and parts of Wales and the South East got updated in this release. A quick look at the map tiles on our systems shows that a full 15% of map tiles were altered in just this one quarterly update.
Processing the Data
From a human perspective, the data processing is not time consuming, we upload the data, and then set the tools running. Then we run some checks on the data before putting it live on Anquet Cloud.
We can’t just deliver the map data to our customers in the format it is delivered. There are 3 main reasons for that:
- The terms of the OS license deem that we must ship the data in a format that only our software can read – i.e. this is to protect copyright.
- The map tiles are too large for smooth operation. As you scroll the map screen, loading some 1km squares to fill the gap is quite quick – loading the next 20km*20km tile would be noticeable on slower machines. On mobile phones these issues are magnified due to the constraints of the devices.
- On the map types where we support our Enhanced Zoom technology, we need to create the Enhanced Zoom map layers.
Most compression systems such as say zip are required to balance speed of compression and decompression against absolute compression ability – you wouldn’t want to wait whilst a photo was compressed to be added to an email for instance.
Our requirements are quite different, we only compress the map images once on Anquet Cloud.
This allows us to tune our customised compression system. The compression step is very slow, but attains a higher level of compression, yet decompress is quick.
The result is smaller map files than other compression techniques would obtain.