NOTE I edited the code because the questioner preferred to choose the track and the points he wanted to measure. The update in the code is the possibility to create a number of point output layers equal to the number of the line features but, if selected in the main dialog, this number could be lower.
I propose a solution using PyQGIS. My idea was creating a certain number of temporary points along the line and then using the nearest neighbor of them from each point of the input points (i.e. your Blackpoints) for measuring the distance from the start/end of the line.
Before posting the code, I need to do some remarks:
- in addition to the point layer and the line layer, my solution requires an additional parameter, called
step
, which is simply the spatial interval for the creation of the temporary points (remember that the accuracy of the result heavily depend from this parameter!);
- all the distances are calculated from the nearest neighbor of each Blackpoint to the start/end of the line (this means that the distance between the Blackpoint and its nearest neighbor is NOT evaluated, but I can immediately and easily add it to the code if needed).
The code will return a point memory layer which stores the same geometries and attributes of the input layer, plus two additional fields:
"DIST_START"
, which stores the distance from the start of the line;
"DIST_END"
, which stores the distance from the end of the line.
This is the code:
##Points=vector point
##Only_use_selected_points=boolean False
##Line=vector line
##Only_use_selected_lines=boolean False
##step=number 1
from qgis.core import *
from qgis.PyQt.QtCore import QVariant
layer1 = processing.getObject(Points)
crs = layer1.crs().toWkt()
layer2 = processing.getObject(Line)
if Only_use_selected_lines:
line_feats = layer2.selectedFeatures()
else:
line_feats = layer2.getFeatures()
for ft in line_feats:
index = QgsSpatialIndex()
tmp_points = {}
line_geom = ft.geometry()
len = line_geom.length()
current = 0
# This layer contains all the points created along the line (it isn't an output)
temp_pts = QgsVectorLayer('Point?crs='+ crs, 'temp_pts' , 'memory')
prov = temp_pts.dataProvider()
# Uncomment the next line if you want to see the 'temp_pts' layer
#QgsMapLayerRegistry.instance().addMapLayer(temp_pts)
while current < len:
point = line_geom.interpolate(current) # Create a point along the line at the current distance
fet = QgsFeature()
fet.setGeometry(point)
(result, feat) = prov.addFeatures([fet])
tmp_points[feat[0].id()] = current
index.insertFeature(feat[0])
current += step # Increase the distance by the step provided
# This layer is the final output which stores the distances from the closest point along the line and the start/end of the line
out = 'output_%s' % (ft.id())
output = QgsVectorLayer('Point?crs='+ crs, out, 'memory')
prov2 = output.dataProvider()
fields = layer1.pendingFields() # Fields from the input layer
fields.append(QgsField('DIST_START', QVariant.Double, '', 10, 3)) # Name for the new field in the output layer
fields.append(QgsField('DIST_END', QVariant.Double, '', 10, 3)) # Name for the new field in the output layer
prov2.addAttributes(fields) # Add input layer fields to the outLayer
output.updateFields()
if Only_use_selected_points:
point_feats = layer1.selectedFeatures()
else:
point_feats = layer1.getFeatures()
for feat in point_feats:
geom = feat.geometry()
attrs = feat.attributes()
nearest = index.nearestNeighbor(geom.asPoint(), 1)
dist_start = tmp_points[nearest[0]]
attrs.append(dist_start)
dist_end = len - tmp_points[nearest[0]]
attrs.append(dist_end)
outGeom = QgsFeature()
outGeom.setGeometry(geom)
outGeom.setAttributes(attrs)
prov2.addFeatures([outGeom])
# Add the 'output' layer to the Layers panel
QgsMapLayerRegistry.instance().addMapLayer(output)
Now let's give an example. Starting from these sample layers (14 points, the line has a length of about 16 km) and using a step
of 1 m:
I obtain this point layer:
with this Attribute Table (the values are expressed in meters):
Best Answer
This is pretty simple so I'll give you the steps but if you are brand new to GIS then you will need to look up more for each individual step but Google is your friend.
Create a new shapefile layer and set it's geometry to 'Line'.
Select the new line layer in the table of contents and toggle editing.
Enable snapping.
Click Add Line Feature and digitise a line from point A to B.
Open the layer's attribute table.
Open the Field Calculator.
Create a new field called 'Distance', make it a decimal number and give it an appropriate length and precision.
In the expression window type '$length'. This will give the field the length of the line in metres (depending on your coordinate reference system). To make this kilometres change the expression to '$length / 1000'.
Under the layer's properties enable editing using the 'Distance' field.
This will now display in the layout view.