One and a half year ago I found PostGIS. I did fast become a fan. Handling spatial data with sql is a wonderful way of doing it. PostGIS also have a great amount of functionality and if something is missing no one will be stopped from creating that functionality. When I realized that I understood that I no longer could complain about a functionality I have missed in other GIS systems. I have done some avenue scripting in Arcview 3.x and solved a lot of tasks that way. But I have missed an easy way to get the information about between which points the distance-function gets that min distance.

Let’s say you are working with linestrings of rivers and you want to know how close a linestring that represents a road is to that river. Ok, the distance-function tells you that the minimum distance is 20 meters. Great, but the next question will be, where. Where is the road only 20 meters away from the river. In a couple of times I have wanted that information and I have always imagined that the information have to be somewhere in there, in the function. To find the minimum distance you first have to identify where to measure, was my thought. That was partly right I found.

That’s the great thing about open source, if you are wondering how it is done the code is there to read. Since I have never studied C before I didn’t have very high expectations of understanding anything. But from quite good commenting and clean structure I successes to put this together

http://www.jordogskog.no/distance.html

The minimum distance between to geometries have to be between two vertexes or between one vertex and one edge. The distance calculation iterated through the vertexes and edges defining the inputted geometries comparing their relations one by one. How to find the distance between two vertexes is just done with the Pythagorean theorem. Little bit worse is it to calculate the distance between one vertex and an edge. Search for “How do I find the distance from a point to a line?” in this link

http://www.faqs.org/faqs/graphics/algorithms-faq/

There is a description how to get the distance from the line to the point. That is the way it was done before. But there is also a description how to identify the point on the edge (line) from where the shortest distance is found. Time for copy and paste. When the overall shortest distance is found the points defining that distance is returned to the user as a line. I found a line being the best way of returning the information because than the user can get both first and last point from that and the distance from the length of the line. The use of this functionality will probably, as described in the beginning be to identify where the minimum distance is found. Let’s say you are sitting on an big Island with your laptop and asking yourself from where you should swim to get the shortest way to shore. Now that problem is solved. For convenience the first point of ST_Shortestline can also be found with function ST_Closestpoint.

From this rewriting a also successes to get maximum distance calculation working, ST_Maxdistance. Then it was natural to also add longest line function which relates to ST_Maxdistance as ST_Shortestline relates to ST_Distance.

To make the symmetry complete I also added ST_DFullywithin. That function returns true if the maxdistance between two geometries is smaller or the same as the inputted last parameter. Just like ST_DWithin but with maximum distance instead of minimum distance.

So as summary

the old functions working with minimum distance, ST_Distance and ST_DWithin has now got a new friend ST_Shortestline and there is also the corresponding functions for max distance, ST_Maxdistance, ST_Longestline and ST_DFullywithin.

I will get back soon and tell about how I found the maybe fastest distance calculation, included in 1.5