Second Newbie question

[Carl Burn]

Hello again,

thanks for the response earlier, have created a test .shp file based on
4 of our image tiles, using gdal. AFAIK this seems to have worked and
has created a set of files in the same folder as the original files.
Iam working in the maplab interface and am trying to create a sample map
with just this as a single layer, I can create the map and the layer but
when I try to generate a preview of it I get the following error message:

*Fatal error*: [MapServer Error]: msCalculateScale(): Invalid image
extent, minx=-1.000000, miny=-1.000000, maxx=-1.000000, maxy=-1.000000.
in *C:\ms4w\apps\maplab-2.2\htdocs\common\wrapper\map_session.php* on
line *241*

I dont have any values in for the extent at present in th emaplab
interface, do I need to enter this information and if so how do I
calculate it?. Sorry if this is a dumb question (again).

Best

Carl