[GIS] Distance to Nearest Hub QGIS not working when ‘hub’ is a line

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QGIS 2.18.20 working in CRS: WGS 84/UTM Zone 35S (EPSG:32735 OTF), with shapefiles of map features (roads, river, point water sources) reprojected from WGS84 EPSG:4326.

All shapefiles have been reprojected (in order to be able to work in metres not degrees). Map features originally from GPS source, input to qgis using lat/long coordinates.

I'm trying to run Distance To Nearest Hub analysis for animal sightings (300+ points in one layer) to the various features in my map (see list above). The analysis works well for the point data (e.g. Source= sighting point, Hub= water source point) but not for the line files (road, river, fence).

For the lines, it just seems to choose a central point to that feature and calculate the distance to that one point from the source (sighting) points (see screenshot).

I've tried adding geom info to the line shapefiles but they just add in length and the 'Distance to nearest Hub' analysis ends the same.

Any ideas?

Best Answer

Using Distance to Nearest Hub tool, I reproduced the same situation as you have posted.

My suggestion is to use a GRASS tool v.distance which can be found under ( Processing Toolbox | GRASS GIS 7 | Vector ).

From is your animal siting points and To is the line layer such as the rivers.

To be honest, using v.distance through QGIS processing toolbox is still tricky and challenging. (You may encounter errors by running it in default setting). My workaround is to remove cat in the 'upload' field and leave it blank.

As you can imagine, this does not give you the distance of each line. To obtain the distance from each animal observation to the nearby river, open the attribute table of Distance (output by v.distance) and use the Field calculator to calculate $length.

Related Solutions

[GIS] Distance to nearest hub output not correct

As alluded to by @BillW, the distance you are getting is in the units of the CRS you are using. The units of EPSG 4326 are degrees. That is, the distance (0.00003) is in degrees.

If you want a distance in metres, use a CRS in metres (like UTM, or perhaps a state plane since it appears you're in the USA). Convert to miles later, if you have to.

QGIS – Calculating Shortest Distance Between Point and Line via Field Calculator and Overlay_Nearest

To get the length of the line, you must not replace make_line by distance, but add the function length to the rest of the expression, then use it in field calculator to calculate the length of the shortest line from between each point and the nearest point on a line.

Thus the whole expression will look something like what follows, where you have to replace the [...] part with the settings that work for you (your initial expression):

length ( make_line ( [...] ) )

Don't forget to add another closing bracket ) at the end when adding the length function.

Screenshot: the dotted lines are created with the same expression as the one you see here, just without the length() part. Be sure to apply this on the points layer and make the right settings: get thecorrect name of the line layer, set Output field type to a numerical format (integer or decimal) and check if the preview at the bottom shows the correct result:

Best Answer

Related Solutions

[GIS] Distance to nearest hub output not correct

QGIS – Calculating Shortest Distance Between Point and Line via Field Calculator and Overlay_Nearest

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