Whether the recorded data is better or worse than imagery is dependent on the particular area being observed. The result is that GPS tracks will frequently have an offset from reality. Limiting factors include weather conditions, atmospheric interference, suboptimal reception (dampening), reflections off objects (in particular, dense urban areas with high-rise buildings), obstructed sky view (when recording tracks from the inside of a vehicle), Selective Availability (dithering of the position) making it less accurate for civilian users and enemies was turned off in the year 2000. As satellites pass overhead, your position may be trilaterated with improved or degraded accuracy over time. The perspective problem can only be countered by taking more overhead pictures per distance, and combining them.
Perspective exists because the camera is only above one certain point at any time. This transformation is not perfect, and may induce distortion and position errors.īecause there is, in any case (even if Earth was flat), perspective in the picture, objects may appear to skew to the side the higher above the ellipsoid they are, so this affects high-rise buildings, mountains, and roads going up/down hills.
AERIAL MAP SOFTWARE
With that, special software can be used to transform it into a projection that is more commonly used with OSM map making. The photo's corner coordinates can usually be derived from height-above-ground and lens properties. Initially, tiled imagery starts life as individual photos, taken from a satellite or airplane, tagged with a center coordinate (which itself is subject to errors, see below on GPS). All of these can be used to create and modify objects, but consider that each may have an offset or other kinds of distortions. 4.3.1 With Imagery Offset Database pluginĪs a background layer within editors, we can use Bing, IRS, Landsat, and some others.4 Matching imagery using different editors.Note that not every high-resolution imagery is also well-aligned one. In some areas there are available aerial imagery and LIDAR data of such good alignment that they are superior to GPS tracks. Aerial imagery may be distorted and shifted relative to real object locations. Aerial images clearly provide useful assistance in the mapping process, but it can be a bad idea to move map objects created by others just to match them to aerial images. The exact placement of roads and other features can be assisted by recording GPS tracks on the ground. Your 'background' local knowledge can contribute, but remember that even that 'knowledge' can get outdated. Self-verified data is the better data to have: the most valuable mapping involves visiting an area and doing ground survey. In fact aerial photos have multiple potential problems, which this article touches upon. What could be better than mapping from the clear, high-resolution aerial photos available in iD, JOSM, and the other editors?
Tracing OpenStreetMap from aerial imagery - a kind of ' armchair mapping' - may seem like a 'no-brainer'.
AERIAL MAP LICENSE
Only use sources for which we have explicit permission or have a compatible license or are out of copyright (often difficult to determine).