Views:

Symptom

A network, possibly imported via some interface or created with some older PTV Visum release, is displayed with a (large) shift against the background map or the background map cannot be shown at all.

Cause

Networks once created without a proper georeference or against a non-georeferenced background map are without any GIS hard to georeference.

Workarounds

1) Try to retrieve the original source, like a paper map and the coordinate system of that map.
2) If the network extents do not exceed about 100 x 100 km, try to transform from an (implicit) local coordinate system into a projected coordinate system (like an UTM zone). Using WGS84 coordinates for this transformation is not recommendable, as in geographic coordinate systems the scale changes from the equator towards the poles.

Consider this workaround to correct at least for a large shift by a translation:
- Open two Visum instances, one (A) holding the network and one (B) showing the background map at the wanted location.
- Make sure B is set to the wanted coordinate system.
- Choose two nodes in the network A, for which you can identify the corresponding position in the background map in B.
- Add these as nodes to B too.
- The coordinate values of these nodes are available in A and B in the window "Quick view" or in the list "Nodes" as node attributes.
- Copy these values to MS Excel.
- Calculate the differences.
In case of some small shift, consider to use the tool "Measure distance mode" of the Network editor to estimate the differences.
- Use the dialog "Transform network co-ordinates" to shift the network to the right location: Network -> Network settings -> Scale -> Transform network co-ordinates
- Set the coordinate system again, and deactivate the option "Transform coordinates in case of changes".
- The node coordinate values can also be used to estimate a scale factor and a rotation, which the dialog "Transform network co-ordinates" can also take as input.
It is recommended to perform the translation, rotation and scale correction in separate steps.

 

Keywords: Coordinate systems/Georeference network/Transform network co-ordinates/Network shifted