PTV VISUM TIPPS & TRICKS:
ADDING A CORRECTLY GEOREFERENCED
GOOGLEMAPS AREAL IMAGE
How to add a correctly georeferenced GoogleMaps aerial image?
The previous issue “How to choose an appropriate coordinate system” addressed spatial
reference systems and how to set them up. This time, we will show how a spatial
reference system can be used to add and use GoogleMaps images within a matching
coordinate system.
Traffic models are sometimes started from scratch, but it is not always easy or costeffective to find a map or map data as basis for the network.
Especially smaller projects can take advantage of aerial images widespread by web sites
like GoogleMaps, GoogleEarth or Microsoft’s Bing. If you click on them, you can get
coordinates in “WGS84”. One could easily take a screenshot and use this as a
background in PTV Visum to create the network from. Of course aerial imagery web sites
display the Earth’s surface as seen from above. PTV Visum however displays WGS84 as
if you where standing on the equator surface. Australia or Germany are “seen” from a 20
to 45 degree angle. How can I use such an image for my traffic model?
 PTV AG Aug-13
Page 1/5
PTV Visum: Tips & Tricks
Proceed as follows:
1.
From the website take the image’s upper left and lower right corner points in
WGS84 .
2.
Create a screenshot and save it as JPG :
3.
Create a new network in PTV Visum. Go to Network  Network Parameters 
System of co-ordinates  Choose “From file”  Click the “…” button  Geographic
Coordinate Systems  World  choose “WGS 1984.prj”
Exit both dialogs with “OK”.
 PTV AG Aug-13
Page 2/5
PTV Visum: Tips & Tricks
4.
Create two nodes, one as “upper left corner “ and one as “lower right corner” and
change their “x” and “y” coordinates to the WGS84 coordinates taken from the web site.
(In fact these are longitude and latitude coordinates in degrees.)
5.
Now change the coordinate system to some matching spatial reference system.
If you can’t find any matching national system, you will always be able to find a UTM
system as follows: Network  Network Parameters  System of co-ordinates  click
“GCS_WGS_1984”  Double click “Geographic Coordinate Systems” to close them 
Projected Coordinate Systems  Utm  Wgs 1984  choose a zone. Leave both
dialogs with “OK” again.
By choosing a metric, Cartesian coordinate system, the coordinates of the two nodes are
transformed. Note that these nodes seem to have moved away from each other in NorthSouth direction. In fact “we” moved from the equator plane to a tangential plane above
our model area
 PTV AG Aug-13
Page 3/5
PTV Visum: Tips & Tricks
6.Now you can add the background image :
a) Click in the Overview menu on “Backgrounds”.
b) Activate the “Insert mode” (the cursor will look like a “+”).
c) Click somewhere between the two nodes.
d) In the dialog “Open: background” choose your screenshot, saved as JPG.
e) Note that the status bar displays the text: “Place object by rubber band. First
corner = top/left.” Now move the cursor onto the upper left node, click and hold
down the left mouse button, drag the mouse to the lower right node and release
the mouse button.
Note how a rectangle was drawn, now replaced by the background file, that is georeferenced correctly.
 PTV AG Aug-13
Page 4/5
PTV Visum: Tips & Tricks
However, not every projected coordinate system will match to the displayed image. This
works for images not exceeding about 10 x 10 km, because the screen projection is only
approximately homogeneous tangential, decaying to the side borders. Putting together
several images beside each other may show these distortions.
After the image has been geo referenced, you can create your model by adding nodes,
links, zones and connectors:
 PTV AG Aug-13
Page 5/5