Tips for Aussies on using overhead imagery
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- Overhead imagery has to be adjusted to fixed ground features for accuracy. In JOSM there is "Adjust the position of the WMS layer" which allows you to line up small areas for accuracy.
- A GPS trace accurate to +/-4m is then as good as imagery with a pixel size of 2 metres. Example imagery of that standard is quickbird, which is the source that Google uses for its in town Google Maps satellite overlay.
- Practical Mapping is to take more traces along the same road, and look at which of those sets of data are the closest. The true road position is likely to be the average of the 2 closest traces.
Yahoo has low resolution imagery which is not particularly good for mapping, and high resolution imagery which is limited in scope to major cities. Set the distance slider on JOSM to about 1km to get high resolution imagery.
Landsat again has two resolutions with much better coverage, but high resolution imagery is not as fine detail as Yahoo's. Landsat imagery available for our use is about 7 years old.
Set the JOSM slider to about 1.5km or less to get the higher resolution imagery.
In Potlatch you can now select between Bing, OpenAerialMap and OSM map layers.