I have a postgis table of points, that are sitting on road centerlines (I've linear referenced them). And the second table with road centerlines themselves. The point table also has a field roadid and a field "offst", that represents the offset in meters (negative values = offest to the left, positive = offset to the right side of the road). The question is how can I calculate a point geometry, that is offsetted perpendicular to road direction, according to the value of the "offst" field?
Answer
In mathematical terms would need to calculate the normal of the road section and unit vector of that normal. The first will find which direction you need to move to get your new point and the latter will help you find out how far to go in that direction (i.e. the translation).
I'm not completely clear on your schema, so difficult to give an sql example, but here's the theory: Points p1 and p2 are the start and end points of your road section, point p3 is "centerline" on that road section and d is your offset distance. The normal to your road section, where dx = p2(x) - p1(x) and dy = p2(y) - p1(y), is the vector (-dy,dx),(dy,-dx). To make this a unit vector you should normalise it by dividing each component by it's length, where length = sqrt((dx * dx) + (dy * dy)). So the unit vector would be (-dy,dx)/length,(dy,-dx)/length. You can then add that unit vector, times by your chosen offset distance, d. So the offset point (to the right of your road) would be offset_point(x) = p3(x) - ( d * ((-dy,dx)/length) ) offset_point(y) = p3(y) - ( d * ((dy,-dx)/length) )
Exactly how this is best achieved in a postgis sql statement depends on how the geometries are stored.
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