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I want to draw a perpendicular line from the mid point of another line. For this, I could not find any function or straightforward approach.

The method I am following is, first I am creating offsets of the line on both left and right sides first with the parallel_offset function of shapely and then finding the mid point of the offset lines and connecting those. This is based on this question post but for GeoPandas data frame data.

The problem I am facing in this is, that for some lines if I am taking a larger offset distance, it is creating an empty line string for the offset. As explained in Shapely parallel_offset returning an empty list, I guess the error is occurring as the offset is vanishing at a certain distance which depends on the curvature of the original line.

I am thinking there must be a way to draw a perpendicular line from another line at any point in Python which can be drawn straightway.

How do I solve this issue?

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  • The "other line" is a simple line consisting of two vertices?
    – BERA
    Jul 7 at 11:58
  • Please Edit the Question to be more specific. If the units are angular (geographic), a perpendicular line isn't actually perpendicular (or even straight).
    – Vince
    Jul 7 at 12:05

1 Answer 1

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Your linked answer can be used for your problem.

In this case Geopandas is just a way to read and write a shapefile. Geopandas geometries are shapely geometries so you can apply the same logic.

Create a function which creates a parallel line, find its centroid then draw a line from your original lines centroid to the parallel lines centroid:

import geopandas as gpd
from shapely.geometry import LineString

df = gpd.read_file(r'/home/bera/Desktop/GIStest/AAAAAAAAAAAAA.shp')

def giveline(linestring, distance):
    "From a input shapely linestring, return a perpendicular linestring with the length equal to intput distance"
    pline = linestring.parallel_offset(distance=distance)
    return LineString([linestring.centroid, pline.centroid])

df['perpline'] = df.apply(lambda x: giveline(linestring=x.geometry, distance=200), axis=1) #Create a column of perpendicular line geometries
df2 = gpd.GeoDataFrame(df, geometry=df['perpline'], crs="EPSG:3006") #Use this to create a new dataframe
df2 = df2[['geometry']]

df2.to_file(r'/home/bera/Desktop/GIStest/BBBBB.shp')

enter image description here

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  • As I have mentioned in the question, this approach is returning a empty LineString when the curve of line is too high and/or the offset distance is high.
    – Badal
    Jul 7 at 14:21

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