[OSM-talk] People's Map

[andrew heggie]

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