Should I draw to the aerial maps?


I’m new to OSM and want to contribute! I noticed that in my area, the various tracks don’t match the Yahoo! aerials. Before I start dragging streets around, can I assume that the aerials have an accurate longitude/latitude, or are the tracks more accurate? In many places, they diverge by a few hundred feet.


Hi, welcome to OSM! Many aerials are very accurate, but they are often misaligned. If you are in the US, I think some of the original Tiger data was based on survey data, while in other areas were hand scribbled on paper. So you’ll need to get a reference GPS track to determine which is correct.

(Assuming Potlatch editor - let me know if you’re using a different editor)
If you don’t have access to a GPS, one thing to check is if anyone else has happened to record a GPS track while travelling through your area. Click on the GPS icon (bottom 2nd from the left) and see if there are any GPS tracks of your area - especially near a major highway or interstate. That will help you figure out which is correct. Assuming that the image is misaligned, you can click on “Help” in the lower left corner, then click “Surveying” at the top, then on the right side there is a good explanation of “Using satellite photos” on the right about how to align the images.

It really depends on the region. My experience in California is that aerial data is way more accurate than most TIGER data. In most other regions that I’ve looked at (not too many), this seems to be the same. But I’ve seen some data in Texas where TIGER data is extremely accurate.

Even if aerial is off, at least in terms of relative positioning, it’s very, very good, so if you align roads to aerial data and somebody passes by with absolute references (real GPS data), it’s a simple matter of moving all the coordinates of a region to the correct reference…

I was going to ask the very same question. The main roads all align perfectly with the aerial photos, but the smaller roads done by tiger are far off in all directions. So it sounds like I should realign them them?

If the main roads are perfectly aligned, then yes, I would align the smaller roads to the aerial imagery. I see this quite frequently.

If there is just a single GPS track, trust the aerial image more. If you have enough tracks to average, then this will show the correct position. I am told that there is a possibility to define an offset if you are using JOSM, but I haven’t tried.
If you are mapping in a hilly area, then don’t trust the aerial images too much: they are often not or not well orthonormized: if a location in a valley is at a correct position, then a location on top of the hill might show some offset, because the photo of this point is taken with some angle. If aerial images are designed to be used for mapping, this deviation is corrected with the knowledge of the elevation of this area and the position of the plane. I don’t think Yahoo! did pay for this work as the images were not intended to be used for mapping.