[GIS] Creating perpendicular line transects in PostGIS

lineperpendicularpolyline-creationpostgis

I have a river network (Line with arrow as shown in image below) and I need to create perpendicular lines to the river at every 15 meters as you walk along it (see image below).

Is there a function in PostGIS capable of doing this?

Best Answer

No clear existing function to do this, but working back from the great suggestions on this post: Making perpendicular lines through each polygon edge using PostGIS?

Assuming you are good with PostGIS syntax, this might help you:

Split the line into 15m segments, like this: https://www.northrivergeographic.com/qgis-split-a-line-at-an-interval
Loop every segment in the newly segmented line and for each segment:
Create parallel lines with ST_OffsetCurve with a desired distance D (can be as longer than the lake/sea as shown above, we will clip them later), on both sides of the line
For every 15m segment, make a new line from the centroid of that segment to the centroid of the corresponding parallel lines with ST_MakeLine
You should have perpendicular lines, spaced 15m extending beyond the river basin/lake
Clip the lines with the sea region

Related Solutions

PyQGIS Lines – How to Draw Perpendicular Lines in PyQGIS?

It is a problem of analytical geometry and the solution was given by Paul Bourke in 1998 (Minimum Distance betweena Point and a Line). The shortest distance from a point to a line or line segment is the perpendicular from this point to the line segment. Several versions of his algorithm have been proposed in various languages including Python as in Measuring distance from a point to a line segment in Python. but there are many others (like Nearest neighbor between a point layer and a line layer with Shapely)

# basic example with PyQGIS
# the end points of the line
line_start = QgsPoint(50,50)
line_end = QgsPoint(100,150)
# the line
line = QgsGeometry.fromPolyline([line_start,line_end])
# the point
point = QgsPoint(30,120)

pt line

def intersect_point_to_line(point, line_start, line_end):
     ''' Calc minimum distance from a point and a line segment and intersection'''
      # sqrDist of the line (PyQGIS function = magnitude (length) of a line **2)
      magnitude2 = line_start.sqrDist(line_end) 
      # minimum distance
      u = ((point.x() - line_start.x()) * (line_end.x() - line_start.x()) + (point.y() - line_start.y()) * (line_end.y() - line_start.y()))/(magnitude2)
      # intersection point on the line
      ix = line_start.x() + u * (line_end.x() - line_start.x())
      iy = line_start.y() + u * (line_end.y() - line_start.y())
      return QgsPoint(ix,iy)

line = QgsGeometry.fromPolyline([point,intersect_point_to_line(point, line_start, line_end)])

and the result is

result

Adapting the solution to your problem is easy,just loop through all line segments, extracting the segments end points and apply the function.

[GIS] Calculate distance from all points to all lines within 10km using FME

I have figured this one out.

I ended up using the NeighborFinder as I initially suspected. The trick is to store all candidate details in a list (I called it NeighbourList).

enter image description here

I then connected that to a ListExploder to create features from all elements in the list. Then I used a tester to filter out the features with a distance of zero. Finally, I used a VertexCreator to add a vertex at the candidate location to make a line.

The results are:

enter image description here

There is still some work needed to get rid of non-perpendicular lines at the bends of the black lines. But generally, the results are good.

enter image description here

Note the gap is where the lines are greater than 10km apart.

Best Answer

Related Solutions

PyQGIS Lines – How to Draw Perpendicular Lines in PyQGIS?

[GIS] Calculate distance from all points to all lines within 10km using FME

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