I'm trying to use QGIS 2.14 to snap a road network to a hexagonal grid, but I'm getting strange artifacts.
I've created a hex grid with MMQGIS, cells are approx 20 x 23 m. I've buffered the road network by 1m and densified it so there is a node every few meters. You can see what I'm trying to achieve below. As you can see, I can get it to work in some cases:-
- blue is the densified road (a buffered line)
- red is the 'hexified' version - this is what I want to find
- the grey is the hex grid
I then used the new Snap geometries feature to snap the nodes to the closest hexagon corner. The results are promising, but there seem to be some edge cases where the line expands out to fill the hexagon (or part of it):-
The reason for the buffer is that Snap geometries doesn't let you snap to a layer whose geometry is different. For example, you can't snap nodes on a LINE layer to points on a POINT layer). It seems to be happiest snapping POLYGON to POLYGON.
I suspect the roads expand out when one side of the buffered road line jumps to one side of the hex cell, and the other side jumps to the other side of the hex cell. In my example, the roads which cross west-east at an acute angle seem to be the worst.
Things I've tried, without success:-
- buffering the road network by a tiny amount, so it remains a polygon but is very thin.
- densifying the hex cells (so there are nodes along the edges, not just at the corners)
- varying the maximum snapping distance (this has the largest effect, but I can't seem to find an ideal value)
- using LINE layers, not POLYGONs
I find that if I change to using just LINE layers, It works for a while, then crashes. It seems to save its work as it goes - some lines have been partially processed.
Does anyone know of any other way to snap points on a line to the nearest point on another line/polygon layer, ideally without needing to use postgres/postgis (although a solution with postgis would be welcome too)?
For anyone who'd like to have a go, I've put a starter QGIS project here on Dropbox. This includes the Hex Grid and Densified lines layers. (The road network is from OSM, so can be downloaded using QuickOSM e.g. if you need to get the original to undensify the roads).
Note that it's in OSGB (epsg:27700) which a localised UTM for the UK, with units in meters.