I was invited to attend a workshop on GIS application to determine land and water use.

They are using Landsat images of 1998 with a pixel sizes of ± 30x40 m and digitize the perimeter of the "green" area with a mouse on the screen (set at 10 pixel snap grid).

The ARC-GIS 9.2 is set at double double precision. I guess about 16 decimal digits. And the area are given to 5 decimal digits precision.

Unfortunately for them I knew the area and the farm they used for an example well. When the area of a centre pivot was calculated as 29.12517 ha I told them that it is incorrect and should be 30.2 ha (It was measured with a tape not so long ago - 310 m)

I tried my best to persuade them to be realistic but to no avail.

What can be done to convince GIS experts?

1## Re: Absurd accuracies in GIS