[postgis-users] project a point onto a line and get x, y, offset of the point of projection

[Stephen Woodbridge]

Hi all,

Someone on the list asked if they could get the point location of where 
on a line object the shortest distance to that line was from a point. 
The distance(line, point) will tell you the shortest distance, but it 
does not project the point onto the line at the point where the shortest 
distance is located.

Then I needed the same thing for a project at Where2GetIt.com and it 
seemed like this functionality should be a part of PostGIS. So attached 
is the code I wrote for our needs. There are some examples of its use. 
It has an assumption that you are working in decimal degrees, but it 
could be modified easily to work in any space. I will leave that up to 
others.

The algorithm is very straight forward. Given a linestring and a point, 
project the point onto each segment of the linestring and save the 
projection of the closest point. Then compute the distance from the 
start of the linestring to the projection point and return an array of 
pointx, pointy, distance.

Paul, please feel free to add this or something like it to PostGIS.

Enjoy, I would be open to any feedback or improvements any might suggest.

-Steve
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