On Saturday 11 April 2009 00:47:46 Tim Waters (chippy) wrote: > > Essentially the first bit burns ground control points into the image the > > second then stretches the image and produces a geoTIFF from it. I visited > > 4 ground control points with my GPS each about 3km apart and at prominent > > points near the corners of each image. I suggest you need farm ore ground > > control points than this at this scale because my georectified image was > > up to 20 metres adrift in some parts. > > Indeed it's quite easy, at the basic level you can use a similar > service such as http://warper.geothings.net or choose from a desktop > GIS, most of them have some way of doing this. These would georectify > images, but we should orthorectify them too which is a bit more > trickier. > > What happens is that the distance from lens to ground is different > over varying terrain, so it doesn't match what a map would be. > > Crudely, imagine taking a snapshot photo from the plane just as you > fly over a mountain top: way down below you'd see the tiny roads, but > most of the frame would be the mountain peak. Of course, > orthorectification is more important with terrain at different > heights, and less so for flat ones.
In my case I am able to get decent altitudes because I use a garmin 60csx to mark my ground control points but I think the work involved to orthorectify is beyond me. Of course this assumes points on the ground rather than the tops of buildings or trees shown in the photo. >From the little I understand from Iván's post it seems that having enough ground control points is a good analogue of orthrectification. In my case the terrain was all withing +-35m height. AJH _______________________________________________ talk mailing list talk@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk
