qgisstyle
I am trying to make a cross hatch polygon fill (example link attached) using the values (defined by law) of 0.1mm for line width and 0.8mm for distance between the lines. But I don't get/see any hatch with such a small values, so I'd like to know if the values less than 1mm are supported anyway or I have to solve this differently?
I'd add my answer as a comment, but since I can't comment (no reputation enough), I'll add as an answer. It seems that you are trying to add centroid connectors to feed into a traffic assignment algorithm. Right? I should say that doing it automatically is not considered best practice in this industry, but it you are going to do it, I'd like to suggest you to make it proportional to the amount of network comprised within the polygon (roadway stock) and the displacement of this network as well.
I'll also mix some of the transportation modelling jargon with the GIS jargon...
First of all, are you considering splitting links or just connecting the new centroid connectors (dummy links) to the existing extremities of the links? Since I don't know what is the case, I'll assume that you want to connect them to the existing extremities. And it DOES make a huge difference in the algorithm if anybody is wondering.
Now is where I think there are two major questions you need to think about: 1st - How to weigh the criteria closest links versus busiest links 2nd - How to spread the connectors through the TAZ so you don't have them all clustered together?
I'll not answer thse questions, so when it comes to the part of the algorithm to actually add centroids, I'll add a single one.
The algorithm I am suggesting is:
Loop through the centroids adding the connectors to a new layer
Now let's put some codes to these steps (I hope the variables names are self explanatory):
Steps 1 & 2:
Let's assume there is TAZ field in both the centroids layer and the TAZ (polygon) layer. Something like this:
connec={}
features = taz_layer.getFeatures()
idx=taz_layer.fieldNameIndex("TAZ")
for f in features:
taz_id=feat.attributes()[idx]
geom = f.geometry()
area= geom.area()
connectors=area*number_of_connectors_per_unit_of_area
connec[taz_id]=connectors
features = centroids_layer.getFeatures()
idx=centroids_layer.fieldNameIndex("TAZ")
for f in features:
taz_id=feat.attributes()[idx]
a=centroids_layer.dataProvider().changeAttributeValues({f.id(): {idx:connec[taz_id]}})
centroids_layer.updateFields()
Step 3:
#Add node layer as suggested in the PyQGIS cookbook
writer = QgsVectorFileWriter("node_layer.shp", "CP1250", fields, \
QGis.WKBPoint, None, "ESRI Shapefile")
idx=taz_layer.fieldNameIndex("TAZ")
for feat in line_layer.getFeatures():
link=list(feat.geometry().asPolyline())
node_a=(link[0][0],link[0][1])
node_b=(link[-1][0],link[-1][1])
new_feat = QgsFeature()
new_feat.setGeometry(QgsGeometry.fromPoint(QgsPoint(node_a)))
new_feat.setAttributes(["how busy is the link", "name of the street", "etc", "etc])"#Here you need to capture and add all the characteristics of the links you want to consider when adding connectors.
writer.addFeature(new_feat)
Step 4:
#This code is very slow, but will suffice #You should look into using spatial index, but this is what I could write by heart...
idx=taz_layer.fieldNameIndex("TAZ")
idx2=node_layer.fieldNameIndex("TAZ")
for feat in node_layer.getFeatures(): #The extremities
for feature in taz_layer.getFeatures():
enclosed = feature.geometry().contains(feat.geom())
if enclosed:
taz_id=feature.attributes()[idx]
a=node_layer.dataProvider().changeAttributeValues({feat.id(): {idx2:taz_id}})
centroids_layer.updateFields()
Step 5:
#Add link layer as suggested in the PyQGIS cookbook
writer = QgsVectorFileWriter("node_layer.shp", "CP1250", fields, \
QGis.WKBLinestring, None, "ESRI Shapefile")
index = QgsSpatialIndex()
for feature in node_layer.values():
index.insertFeature(feature)
for feature in centroids_layer.values():
geom=pt.feature.geometry()
pt = QgsPoint(geom.asPoint())
nearest = index_from.nearestNeighbor(pt, 100) #100 is a guess, you might need to do more...
#Nearest contains the IDS of the nodes, so it is wise to build dictionaries or numpy arrays with all the relevant criteria (TAZ, if it is busy or not, etc) before you even start looping through centroids
for i in nearest:
if criteria1(i) and criteria2(i) and ... and criterian(i):
P=[pt, node_i.geometry().asPoint()]
line_geom=QgsGeometry.fromPolyline(P)
new_feat = QgsFeature()
new_feat.setGeometry(line_geom)
new_feat.setAttributes['Whatever','attributes','you','want']
writer.addFeatures([new_feat])
Well... It ended up being an answer longer than I had anticipated, but it should get you started... And just as a warning, I did NOT test this code, so it is likely to have typos and other mistakes...
I got a pretty good result using GRASS v.generalize from the Processing Toolbox using default values:
Some lines remain untouched.
Best Answer
First go into your layer properties and then style. Hit the plus button below Symbol Layers; it should load up a simple fill symbol. In the symbol layer type section on the right select "Line Pattern Fill" and then you can set the angle, line width etc. Repeat this so you have two "Line Pattern Fill" symbol layers and just set them up with the same attributes EXCEPT have the angles set perpendicular to one another.
Hope that helps.