[GIS] Calculate distance between 2 points following a path in QGIS

distancelinepointqgisrouting

I'm currently working on bird's data. I have coordinates of observation points and I'd like to measure the distance between points following a path. The given bird is probably moving from point to point therefore euclidean distance between 2 points is probably far from real distances.

Let's imagine I have 3 sites : A,B and C. B is between A and C: For going from A to C, the bird walk A → B → C so the distance AC is given by the sum of distances AB + BC.

I'm looking for the mean of getting quickly all possible distances: AB, BC and AC. I'm used to QGIS.

Of course, I could simply do the sum of euclidean distances myself but I have 15 points so it would be so long to get the distances for all points' couple.

Do you have an idea of how to do it automatically ?

Best Answer

Another approach by a Virtual Layer.

Given you are working on avian movement, I assumed the data is on geographical coordinates (LatLong).

1) Created a dummy data.(The layer name is PointLayer1)

2) Layer | Add Layer | Add Virtual Layer

Import the PointLayer1
Write query to measure distance between each stop:

Query:

SELECT a.loc AS A, b.loc AS B, ROUND(st_distance(a.geometry, b.geometry, 4326) / 1000, 3) AS EACH_km
FROM PointLayer1 a
INNER JOIN PointLayer1 b ON a.loc = b.loc -1
ORDER BY a.loc

Which gives a virtual layer (please open the attribute table) with each flight distance (kilometers) between A and B.

As you need the total flight length, modify the syntax slightly to:

SELECT total(EACH_km) AS TOTAL_km
FROM 
(SELECT a.loc AS A, b.loc AS B, ROUND(st_distance(a.geometry, b.geometry, 4326) / 1000, 3) AS EACH_km
FROM PointLayer1 a  INNER JOIN PointLayer1 b
ON a.loc = b.loc -1
ORDER BY a.loc)

It will give you something like:

When you use the syntax, please replace the PointLayer1 by your layer, and loc by your field to identify the order of stop points.

3) Run scenarios

You can modify order of points by changing loc field. When you change the loc then the virtual layer will update the calculation.

Related Solutions

[GIS] Calculate min distance between points in PostGIS

It's not a simple problem, it involves some kind of forced iteration over the set of candidate points. This chapter from the workshop shows a similar problem, but not exact (your problem is slightly easier)

http://postgis.net/workshops/postgis-intro/advanced_geometry_construction.html

The nearest neighbor searching chapter from the workshop shows the tools you might use to do an index-assisted approach with some external loop driving the query

http://postgis.net/workshops/postgis-intro/knn.html

If your points have a distinct id and you know a distance tolerance (9999) they will all fall within, a self-join and use of the "DISTINCT ON" filter will get you the answer in one go.

WITH unfiltered AS
(
  SELECT t1.id AS id1, t2.id AS id2, ST_Distance(t1.geom, t2.geom) as dist
  FROM t t1, t t2 WHERE ST_DWithin(t1.geom, t2.geom, 9999) AND t1.id <> t2.id
  ORDER BY t1.id, ST_Distance(t1.geom, t2.geom) ASC
)
SELECT DISTINCT ON (id1) id1, id2, dist FROM unfiltered;

It first gathers the candidates combinations of points, and sorts them by distance. Then the "distinct on" filter strips out just the first member of each candidate group, which conveniently is the closest, thanks to the pre-sorting.

[GIS] How to calculate the distances between the points in sequence automatically

An advanced method that will work on all levels of ArcMap (must be 10.1 or above though) would be to read the geometry of each point and project it on the fly to compute the distance between the features. This is a good read about how the Near tool works.

The following code does this (all you'll need to do is find a good PCS for your dataset):

import arcpy
from math import hypot    

shp = "path to shapefile"
#Create a spatial reference object for the PCS...change as needed
spatref = arcpy.SpatialReference(26915) #UTM 15N

#Get coordinates of each feature (in PCS) and store it as a list of tuples.
coord = [x[0] for x in arcpy.da.SearchCursor(shp, "SHAPE@XY",
                                             spatial_reference=spatref)]

#Compute distances between features.
distances = [hypot(coord[i+1][0]-coord[i][0],
                   coord[i+1][1]-coord[i][1]) for i,_ in enumerate(coord[:-1])]

#Insert a 0 in the first position since it has nothing to measure against
distances.insert(0, 0)

#Add field to store distances
arcpy.AddField_management(shp, "DIST", "DOUBLE")

#Write distances back to shapefile.
with arcpy.da.UpdateCursor(shp, "DIST") as rows:
    for j,row in enumerate(rows):
        row[0] = distances[j]
        rows.updateRow(row)

Best Answer

Related Solutions

[GIS] Calculate min distance between points in PostGIS

[GIS] How to calculate the distances between the points in sequence automatically

Related Question