Which point is right?

Our new GPS location recording device

Our new GPS location recording device

In the run up to our new OxPoints system being launched, we’ve started to update some of our data and at the same time we’re attempting to make it just a little bit more accurate. Our basic data will be comprised of the main pedestrian and disabled (if different) entrances to each of the buildings we choose to record. Unfortunately recording the location of entrances is fairly difficult using standard GPS devices and our experience with a variety (more on that in another post) showed that nothing we already owned did the job particularly well. So we set our sights on aquiring a new device, namely a Garmin eTrex Vista HCx which would hopefully give an appropriate level of accuracy by means of having a high sensitivity chipset, being able to automatically average multiple readings and being EGNOS/WAAS enabled.

So far the results have been relatively consistent and a fair bit easier to record over previous devices but we now have a different problem – which map is correct? Below are screenshots of the same latitude and longitude (51.75982, -1.25724)  shown on different mapping sources. I was actually standing on the red cross as shown in the first picture.

Now the question is, do we align our co-ordinates with a satelitte imaging source such as Google or Microsoft or do we leave our data ‘raw’? It seems that leaving our data raw seems to be the ideal method of ensuring consistency, but then the problem then lies with our users who will probably use a third party mapping source to overlay our data onto thus giving locations which are not entirely correct. We could of course correct for this verbosely by adding metadata to each location, but it seems to defeat the point. Opinions would be welcome!

Google Earth Imagery

Google Earth Imagery for 51.75982, -1.25724 (matches Google Maps)

Microsoft Aerial Imagery for 51.75982, -1.25724

Microsoft Aerial Imagery for 51.75982, -1.25724

Yahoo Aerial Imagery for 51.75982, -1.25724

Yahoo Aerial Imagery for 51.75982, -1.25724

Multimap Birds Eye Imagery by Microsoft

Multimap Birds Eye Imagery by Microsoft

Map Quest Aerial Imagery for 51.75982, -1.25724 (not very detailed)

Map Quest Aerial Imagery for 51.75982, -1.25724 (not very detailed)

2 Responses to Which point is right?

  1. Wulf says:

    How accurate do the points need to be? I know you’ve just bought a new device to get very accurate data but, when it comes to somebody viewing this on a map, is the margin of error enough to serious inconvenience them? Any of the maps shown would get them to the entrance to the car park.

    If you do want pinpoint accuracy and agreement between mapping services, is there a transformation you could run the data through to translate from one to another or does each service show a random level of drift as it moves between different map tiles?

  2. Tim Fernando says:

    In an ideal world the accuracy would be around 0.5m, allowing us to pinpoint a specific door on, for example a row of terraced houses. It’s arguable that this level of accuracy is strictly needed at the moment, but if it is at all possible to do so, we’d rather do this now than have to do it all again later. Saying that, it is likely that we will continue to refine the data we have in the system anyway so perhaps it’s just an inevitability.

    As for the transformation – we’re not entirely sure yet, I’m going to go off and do some more testing this morning and I will try and blog about the results I find and see if they are consistent. If they are consistent, then there is a good chance we may be able to add a correction vector to each value depending on the map tile set which is being used but this also has the possibility of going wrong if the source creator decides to change their map tiles at any time.

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