The distance was relative: it was to be the middle of the segment of the line where the middle of the line occurred. Was that clear? A line can have several segments. The segment that interests us is the one on which the line hits half of its total distance. We want the point at the middle of that segment. We want it real bad!
A quick look at the OGR API tells us there's a Value method for OGRLineString. But that's not exactly what we want, and there's no Python binding anyway. So, God, what am I gonna do?
"Don't panic." a calm voice told me. "You have good friends and they've dragged you out of a bigger mess than this one before."
So I calmed down and started coding, and very soon I was feeling better. Here's is the piece of code that flowed from my fingertips:
for line_feat in layer: geom = line_feat.geometry() num_p = geom.GetPointCount() current_distance = 0.0 half_distance = geom.Length() / 2 for i in xrange(num_p-1): if i+1 < num_p: line = ogr.Geometry(ogr.wkbLineString) line.AddPoint(geom.GetPoint(i),geom.GetPoint(i)) line.AddPoint(geom.GetPoint(i+1),geom.GetPoint(i+1)) segment_len = line.Length() if (current_distance + segment_len) > half_distance: x_0 = line.GetPoint(0) y_0 = line.GetPoint(0) x_1 = line.GetPoint(1) y_1 = line.GetPoint(1) d_x = (x_1 - x_0) / 2 d_y = (y_1 - y_0) / 2 x_n = x_1 - d_x y_n = y_1 - d_y createPoint(x_n,y_n) break current_distance = current_distance + segment_len
- Get the half-length of the total line feature
- Obtain the points from which you reconstitute each segment composing the line
- You calculate the length of these segments and sum them up, when you've exceeded the half-length of the total line, you know you're holding your precious segment
- You calculate the point at the middle of that segment - that's your point!