OpenStreetMap

Teversham Parish progress

Posted by jpsa on 2 January 2012 in English (English)

I've been working on Teversham Parish, including the Foxgloves estate adjacent to Cherry Hinton.

I started by improving the accuracy of the existing mapping, correcting a few errors and adding missing detail as I go. My basic strategy is to gather enough GPS data to be able to calibrate the alignment of the aerial imagery, but then to take most of the detail from the imagery.

Firstly I'm gathering a few well averaged waypoints: I'm following Garmin's advice here, and averaging 16 separate samples, taken at least 90 minutes apart, with each sample averaged for 30 seconds. I'm picking points which are clearly identifiable on the aerial imagery, and which have a good clear view of the sky (and specifically with a southern horizon good enough to pick up EGNOS corrections).

Secondly, I'm gathering many track logs (typically ten or more) along a few routes within the area of interest. These act as a cross check on the position of the averaged waypoints, and give me an idea of how accurate (or at least reproducible!) the GPS data is.

Then I'm firing up Potlach2, using space-drag to bring the imagery into alignment with my GPS data, and then adjusting the OSM data to match the (now calibrated) imagery. That's pretty much done now for Teversham Parish, though I may make a few slight adjustments out towards Fulbourn Tesco once I've finished getting another good reference point out that way.

I'm also checking against ground truth by visiting the mapped areas.

I just wish there was a way to leave a note for other mappers to warn them that the raw Bing data is a few metres out, so that they don't come along and unknowingly undo all the good work! I can't even work out what the best thing is to put in the source tag: I've been using "source=survey;bing" which matches some recommendations I found in the help pages (though I noticed that Potlach2 does warn me that I might want to think twice about giving the tag multiple values).

The new project is to get house address information entered. For now, at least, I'm just putting a building node down for each address and filling in addr:housenumber and addr:street. I couldn't find anything in the documentation saying where on the building the node should go, so I've been trying to put the nodes at the front door. Drawing full building outlines seemed too much work for now!

I'm about halfway through doing Teversham Village, and should finish this fairly soon, but it'll be longer before I can face doing the Foxgloves estate!

Location: 52.201, 0.189

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