Can I assume orthoimagery is the best for tracing buildings?
Can I assume orthoimagery is the best for tracing buildings?
I'm trying to correct local buildings on OSM. I've noticed that some of the buildings were traced before according to one set of satellite images, but are off according to others. One of the options for a background while editing that I've got is called orthoimagery. Can I assume that that is the best set of satellite images for tracing buildings from?
Short answer: yes
Pretty much all of the imagery options available on ID are orthoimagery, since that just means it's been corrected to remove distortions from camera angle, lens, and topography.
Unfortunately, that doesn't always make them perfectly accurate. The only good way to check if a particular dataset is accurate is to record GPS along a feature identifiable in the imagery and check out the offset.
Usually the best one to use is a more local survey that covers just the city (usually only done for larger cities). Also ones where the area you're tracing has a more "overhead" camera angle (hard to trace when the top of tall buildings are very offset from their base).
I think the RapID editor automatically provides a few additional local imagery sets, depending on where you are working.
In some places there are government basemaps available as well. These can be extremely accurate, up to centimeter precision. Of course they can be outdated or have mistakes too, and not all are as precise. So it's hard to answer the question without knowing the location. Always vest to consult with local mappers about things like this.
@BitSound @openstreetmap
Depends on the layer. Some of the ones described as ortho imagery are provided by government sources and are really closely aligned to very well known survey points. Others aren't.Even if you do have a layer done to a very high standard it could be older than some of the lower accuracy ones so there will be individual buildings that have been replaced or remodelled and the later source should be used.
Usually roads have lots of gps tracks, so you can adjust the image offsets to match the gps tracks.
You should prefer orthoimages from national or federal mapping agencies. They will often diverge from each other, as they have different resolutions and quality standards. The process of correcting aerial images for topography is called or tho rectification, and a digital elevation model (DEM) is required for that. It has several downsides, one is that there is more distortion in high object like buildings. A quality standard in Germany for example is the TrueDOP (DOP = Digital Orthophoto). You can read more about it in this article if you are interested.