Measure aerial distance between postal codes

Question

Need some help to calculate the aerial distance between the center of a set of postal codes and the center of one central postal code within that region. I want to duplicate this for six different regions. My shapefile is based on PC4 (postal code, 4 digits) of the Netherlands. The measure tool is currently greyed out and I am not able to select a coordinate system at the Data Frame properties tab. Any help would be much appreciated!

Casper

Screenshot of the coordinate system problem:

http://i.imgur.com/HuZrYMQ.png

CasperWesterveld · Answer

With some help of the helpdesk at my university I managed to add a defined coordinate system. I am now able to use the measure tool. I have only one question remaining. Is there a fast way to measure the distance from all the different postal code zones to the center zone in the region at once and to duplicate this strategy to five other regions as well? So I don't need to measure it one by one?

DanPatterson_Retired · Answer

Check the layers... hopefully one of them has a defined coordinate system.  All the data frame properties means is that there is no defined coordinate system for it, but if on of you layers has one, then you can use it, so just right-click on a layer, go to properties and find out what one of them is.  If none of them have a defined coordinate system, then you need to find one.

CasperWesterveld · Answer

No unfortunately the coordinate system can't be defined as can be seen below. I find it very strange because I have worked on a PC from my university with all the licenses available. I don't understand why I am not able to select a coordinate system and after that measure the aerial distances.