Is it possible in QGIS 3.x to click in two places on the map and to get back the X and Y distances between these two locations (latitude and longitude differences).
I was hoping there is a plugin that does that but couldn't find it.
This would be very helpful.
Here is an example code which subclasses QgsMapToolEmitPoint to create a custom map tool which should do what you want. It also implements the QgsRubberBand class to draw a temporary line on the map canvas to track your start and endpoints. You can paste this script into a new editor in the python console and click 'Run script'. You will see the cursor is now a cross. When you click once on the map canvas you can then move the cursor around and you will see a red line following your cursor like this:
On the second canvas click, you will get a dialog box showing the x and y difference values in the current project CRS map units like so:
Because we are using a QT Dialog Box with a Text Edit widget to display the results, we are now able to copy and paste the result values. The text edit is set to read-only so you can't inadvertently edit the results.
When you click OK on the dialog box, the rubber band will be reset and you can keep clicking your two points as many times as you want.
To deactivate this custom tool, just select a different map tool from the map navigation toolbar.
Note: In this code, I subtract the start x and y from the end x and y. This returns a positive difference value if the x or y value increases and a negative difference if x or y value decreases. For example- if the endpoint is north-east of the start point you will get positive difference values to reflect the increase in latitude (y) and longitude (x). You could change the calculation if you want but this makes sense to me.
class XY_difference_tool(QgsMapToolEmitPoint):
def __init__(self, canvas):
self.canvas = canvas
QgsMapToolEmitPoint.__init__(self, self.canvas)
self.start_point = None
self.rubber_band = QgsRubberBand(iface.mapCanvas())
self.rubber_band.setColor(QColor(Qt.red))
self.rubber_band.reset()
def canvasPressEvent(self, event):
project = QgsProject().instance()
units = project.crs().mapUnits()
str_unit = units.baseClass().encodeUnit(units)
if self.start_point is None:
self.start_point = self.toMapCoordinates(event.pos())
start_X = self.start_point.x()
start_Y = self.start_point.y()
else:
start_X = self.start_point.x()
start_Y = self.start_point.y()
end_point = self.toMapCoordinates(event.pos())
end_X = end_point.x()
end_Y = end_point.y()
self.result = result_dialog()
self.result.result_box.setText('X Differential: {} {}\nY Differential: {} {}'.format(end_X - start_X, str_unit, end_Y - start_Y, str_unit))
self.result.show()
self.result.btn_ok.clicked.connect(self.msg_accepted)
def msg_accepted(self):
self.result.close()
self.rubber_band.reset()
self.start_point = None
def canvasMoveEvent(self, event):
if self.start_point is not None:
self.rubber_band.reset()
v1 = self.start_point
v2 = self.toMapCoordinates(event.pos())
self.rubber_band.addPoint(v1)
self.rubber_band.addPoint(v2)
class result_dialog(QDialog):
def __init__(self, Parent=None):
QDialog.__init__(self)
self.setAttribute(Qt.WA_DeleteOnClose)
self.setModal(True)
self.setGeometry(150, 150, 350, 100)
self.setWindowTitle('XY Difference Result')
self.result_box = QTextEdit(self)
self.result_box.resize(350, 50)
self.result_box.setFont(QFont('Arial', 10))
self.result_box.setReadOnly(True)
#self.result_box.setText('Test line 1\nTest Line 2')
self.btn_ok = QPushButton('OK', self)
self.btn_ok.move(255, 70)
T = XY_difference_tool(iface.mapCanvas())
iface.mapCanvas().setMapTool(T)
Create a scratch layer (line).
Apply a label with this expression:
'dy = ' || to_string( y(start_point( $geometry))-y(end_point( $geometry))) || ' , dx = ' || to_string( x(start_point( $geometry))-x(end_point( $geometry)))
It will look like: dy = -0.7004608294930876 , dx = -0.9370199692780339.
You can change the text as you want, eg if you want it to look like Change in Latitude = -0.7004608294930876 degrees, Change in Longitude = -0.9370199692780339 degrees, use
'Change in Latitude = ' || to_string( y(start_point( $geometry))-y(end_point( $geometry))) || ' degrees' || ', Change in Longitude = ' || to_string( x(start_point( $geometry))-x(end_point( $geometry))) || ' degrees'
Use the round() function if you want fewer decimal places.
Optional: if you want to see the coordinates of the start and end points, change to rule-based labeling and create two new rules:
First rule, label with this expression:
y(end_point( $geometry)) || ' , '|| x(end_point( $geometry))
Notice that the coordinate order; Y comes first because Y is latitude and X is longitude. If you wanted labels for easting and northing, reverse the order.
Data-defined placement for first rule: X = x(end_point( $geometry)) and Y = y(end_point( $geometry))
For the second rule, duplicate the first rule and substitute start_point every place where end_point was used.
When you want to measure the change in lat/long between two points, simply create a line feature between those two points. Because this is a scratch layer, any line features you create will be automatically deleted when you close the QGIS project. You can also clear them at any time by rolling back edits on the layer.
If you want to be able to copy the dy and dx values, use the field calculator to add a virtual field for dy and one for dx. Copy the dy and dx values from the Identify tool.
