# postgis, extrapolate a line

I'm trying to extrapolate from a line segment to find a point on the line but a 3rd of the way 'back', i.e. trying to find point `new`, given points `A` and `B` below:

Given a line, I can interpolate it to get a position at any particular percentage along it:

``````=# select st_line_interpolate_point(
st_makeline('0101000020E6100000300DC347C49418C03EE8D9ACFAA44A40',
'0101000020E6100000FB743C66A03218C0CDCCCCCCCC7C4A40'),
0.333);
0101000020E6100000ED45B41D537718C069C6A2E9EC984A40
``````

I tried entering a negative number to find a point along the line in the opposite direction, but that fails as the interpolation argument has to be in the range [0, 1]

I thought about first scaling the line, but that doesn't use the centre of the line as the origin, so is useless for my purposes.

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Another way I have solved a similar problem previously is to break it down into the following steps.

``````-- get the points A and B given a line L
A := ST_STARTPOINT(L);
B := ST_ENDPOINT(L);

-- get the bearing from point B --> A
azimuth := ST_AZIMUTH(B,A);

-- get the length of the line A --> B
length := ST_DISTANCE(A,B);
newlength := length + (length * (1/3));   -- increase the line length by 1/3

-- create a new point 1/3 as far away from A as B is from A
newpoint := ST_TRANSLATE(A, sin(azimuth) * newlength, cos(azimuth) * newlength);
``````

EDIT: fixed the assignment of newlength so that it is 1 1/3 the length, rather than 1/3 and switched A & B around to match diagram.

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We have a winner! Much more comprehensible. – EoghanM Sep 10 '12 at 15:34
This is pretty cool – Nathan W Oct 17 '12 at 5:29
Thanks. I originally had this snippet from some work I was doing manually interpolating between contour lines - turned out it was a waste of time what I was trying to do with the contours, but glad this this snippet helped someone else out! :) – Jayden Oct 17 '12 at 6:29

Have solved it with:

``````F = 1.3333
st_affine(A, F, 0,
0, F,
(F-1)*-st_x(st_line_interpolate_point(st_makeline(A, B), 0.5)),
(F-1)*-st_y(st_line_interpolate_point(st_makeline(A, B), 0.5))
)
``````

Explanation:

(2-d) Scale the start point by a factor of 1.3333, taking the midpoint of the line segment as the origin for scaling.

Get out the graph paper!

http://en.wikipedia.org/wiki/Affine_transformation

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