[GIS] Measure the distance between 2 points

csvdistancepointqgis

This is my first time using QGIS(2.14.18), I construct this map, but I don't know how to measure the distances (D1, D2, D3, Dx …) between the circles and the stars. So I have 2 layers (based in 2 CSV files), with thousands and thousands of lines, the image below is just a test.

The distances I need are only between stars and circles, never between star to star or circle to circle. One star can have infinite circles, but a circle can only have one star link. In the two CSV files exist a key ID that stars and circles share with each other, but I don't know how to use it in QGIS to solve this problem.

test 33

I have tried using the distance matrix in QGIS, but I don't know if I'm doing this right. I think the distance matrix is giving redundant results, like 2 times the same distance or calculating the distances between stars and circles with no link between them.

I need these distances in meters or kilometers, not degrees.

Best Answer

You can do this directly in QGIS using a join and the field calculator.

Do your tables have X/Y information in them? Yes, then go to step 2 else do step 1 first.

Create two new fields in each table and populate with the X and Y coordinates of each point. You can use the field calculator in QGIS and the $x and $y functions to calculate the values.

Join your stars table to the circles table using the keyID. This will give you pairs of points with a structure like circle_id, circle_x, circle_y, stars_star_id, stars_star_x, stars_star_y. Each circle will have a star. (in the image below the distance field has already been calculated. See step 3 below.)

Now you can create a new virtual field in the table called distance and type double and use the field calculator to populate it with the distance to the star from each circle:sqrt(abs(to_int(stars_star_x) - to_int(circle_x))^2 + abs(to_int(stars_star_x) - to_int(circle_y))^2)

If you want the lines on the map as well as the distance then the MMQGIS plugin mentioned above has the hub tool.

Or you could do it in PostGIS as answered here: How to create spider diagrams (hub lines) in PostGIS?

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[GIS] How to measure the distance of multiple points to a line

Try this: from the attribute table of your point file add a new field (real number) and use the following formula:

distance( $geometry , geometry ( get_feature( 'line-file','field',name-of-line))))

Were 'line-file' is the file where the line is stored, 'field' is a field in the line-file table, and name-of-line is the values of that field for your line. This formula gives only positive distance.

QGIS – How to Measure Distance Between Points Based on Elevation

Achieving this goal is somewhat a basic task in GIS, however the method in QGIS might not be trivial. Your best chance is to use GRASS's r.walk function, which creates an anisotropic cost surface (dem+slope+other factors).

First, you have to create a friction surface as an input to r.walk. In your case it can be a single-valued raster (1.0) matching the extent of your DEM. You can create it with r.mapcalculator with the formula: A*0+1 where A is your DEM.

Next, you have to select a set of starting points from your CSV. These are the points, the accumulated cost surface will be calculated from. You have to create an individual cost surface from every starting point. It might be smart to define the ending points associated with every starting point in this step (in individual layers off course). After, you can run r.walk with the created inputs. The starting points can be in a single layer, you can iterate through them with the green arrow in the dialogue box.

Now in an ideal case, you have the cost surfaces and the ending points for every cost surface. In theory you could find the least cost paths with r.drain, but for me, it ended up in an error (python couldn't import the QgisRaster library). If you run in the same issue, you can go with SAGA's "Least cost paths" algorithm. It will create a point and a line layer for every ending point with the cost surface (use the iteration button again). After you have all the lines, you can merge them into a single shapefile with SAGA's "Merge shapes layers" tool.

This method can be very slow with the increment of points, so if you have a lot of them, you might try to automatize the method with python. It will be still a lot of time to calculate (especially the cost surfaces), but you don't have to create tons of ending point layers manually.

Best Answer

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