See attached map file with simplified example. Contains 1 Map with two layers:

- Background = Virtual Earth image - used to "Create" the Map (default projection) - Drawing layer with two points added later (custom projection, offset from UTM53N)

Open the Drawing layer by itself & measure the distance between the points = 20930.4015m (correct for the point coordinates).

Open the Map & measure the distance between the points = 26273.5641m (WRONG)

Struck this problem in a map where the overlain objects were generated by a complex simulation model. Error in the reported distances had me searching for a bug in my simulation program for several hours :-( Penance for inadequate faith in my own coding?

David

Attachments:
Virtual Earth - Distance measurement error.map

This is expected behaviour read the Tracker help topic, here's an excerpt relevant to the projections you have in your example map:

'At times the difference between Euclidean and ellipsoidal calculations will be great even over relatively small regions. For example, when using the tracker tool with Mercator projections (which do not preserve distances accurately away from the Equator) there will be great differences between the tracker and the CTRL-tracker measurement. This effect may be very noticeable when using image servers to provide linked images in Mercator projection. When making measurements in such cases use the CTRL-tracker numbers.'

Chat with Manifold users on IRC Server: irc.freenode.net, Channel: #planetmanifold, IRC FAQ

Many thanks

Was not aware there was an issue. CTRL-tracker does indeed fix the problem.

David

who knew? thought i had messed up somewhere-