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QgsGeometry.length() giving wrong result?
Is it possible to programmatically add calculated fields?Calculate line lengths with Python in QGISWhy can't I multiply values from column named 'length' using field calculator via PyQGIS?Why would having a reference string “boh@10” cause a crash?new vector layer's field based on other layers attributes - “for”cycle errorCalculate Expression with Python?how to get first and last point of MultiLineString-objects in qgis3Adding new field with expression in pyqgis
.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty margin-bottom:0;
I have a vector layer with Multilinestrings and used the field calculator to add a field ("length") with the calculated $length of each feature. I know the field values are correct. Now, if I calculate the length of the features with PyQGIS, I get different, wrong results. Does anybody know, what could be the reason? I don't know where to start looking for the error.
Code example:
layer = iface.activeLayer()
feature = next(layer.getFeatures())
print(feature.geometry().length() / feature['length'])
Result: 1.67
pyqgis qgis-3
edited Aug 16 at 20:35
PolyGeo♦
54.9k17 gold badges86 silver badges258 bronze badges
asked Aug 16 at 10:53
lueholueho
1309 bronze badges
add a comment |
I have a vector layer with Multilinestrings and used the field calculator to add a field ("length") with the calculated $length of each feature. I know the field values are correct. Now, if I calculate the length of the features with PyQGIS, I get different, wrong results. Does anybody know, what could be the reason? I don't know where to start looking for the error.
Code example:
layer = iface.activeLayer()
feature = next(layer.getFeatures())
print(feature.geometry().length() / feature['length'])
Result: 1.67
pyqgis qgis-3
edited Aug 16 at 20:35
PolyGeo♦
54.9k17 gold badges86 silver badges258 bronze badges
asked Aug 16 at 10:53
lueholueho
1309 bronze badges
add a comment |
I have a vector layer with Multilinestrings and used the field calculator to add a field ("length") with the calculated $length of each feature. I know the field values are correct. Now, if I calculate the length of the features with PyQGIS, I get different, wrong results. Does anybody know, what could be the reason? I don't know where to start looking for the error.
Code example:
layer = iface.activeLayer()
feature = next(layer.getFeatures())
print(feature.geometry().length() / feature['length'])
Result: 1.67
pyqgis qgis-3
edited Aug 16 at 20:35
PolyGeo♦
54.9k17 gold badges86 silver badges258 bronze badges
asked Aug 16 at 10:53
lueholueho
1309 bronze badges
I have a vector layer with Multilinestrings and used the field calculator to add a field ("length") with the calculated $length of each feature. I know the field values are correct. Now, if I calculate the length of the features with PyQGIS, I get different, wrong results. Does anybody know, what could be the reason? I don't know where to start looking for the error.
Code example:
layer = iface.activeLayer()
feature = next(layer.getFeatures())
print(feature.geometry().length() / feature['length'])
Result: 1.67
pyqgis qgis-3
pyqgis qgis-3
edited Aug 16 at 20:35
PolyGeo♦
54.9k17 gold badges86 silver badges258 bronze badges
asked Aug 16 at 10:53
lueholueho
1309 bronze badges
edited Aug 16 at 20:35
PolyGeo♦
54.9k17 gold badges86 silver badges258 bronze badges
asked Aug 16 at 10:53
lueholueho
1309 bronze badges
edited Aug 16 at 20:35
PolyGeo♦
54.9k17 gold badges86 silver badges258 bronze badges
edited Aug 16 at 20:35
PolyGeo♦
54.9k17 gold badges86 silver badges258 bronze badges
edited Aug 16 at 20:35
PolyGeo♦
54.9k17 gold badges86 silver badges258 bronze badges
54.9k17 gold badges86 silver badges258 bronze badges
asked Aug 16 at 10:53
lueholueho
1309 bronze badges
asked Aug 16 at 10:53
lueholueho
1309 bronze badges
asked Aug 16 at 10:53
lueholueho
1309 bronze badges
1309 bronze badges
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
The $length expression function states that
The length calculated by this function respects both the current
project's ellipsoid setting and distance unit settings. For example,
if an ellipsoid has been set for the project then the calculated
length will be ellipsoidal, and if no ellipsoid is set then the
calculated length will be planimetric.
QgsGeometry.length on the other hand says
Returns the length of geometry using GEOS
Sadly this is not very helpful if you don't know what GEOS means. GEOS is the underlying geometry engine and it has no idea about coordinate systems, ellipsoids and all that spatial stuff. It is purely about geometry. This means QgsGeometry.length will consider all coordinate values of the geometry as flat cartesian coordinates. You might know they are geographic/ellipsoidal, GEOS does not.
If you are in a length-preserving projection with meaningful units (meters), the result will (almost) be the measurement as if done in the real world. But in all other cases, the results from this function are rather useless.
answered Aug 16 at 11:05
bugmenot123bugmenot123
4,55214 silver badges43 bronze badges
add a comment |
Your layer's coordinate system is probably a Geographic one (meaning Ellipsoidal here), therefore the distance calculated in degree dimension. To solve this issue, first you have to a pick a proper Projected coordinate system, and save the layer in that coordinate system (Save as, change CRS).
You can find a proper Projected coordinate system at https://epsg.io, e.g. search for your country.
After the projection, the lenght() method's result will be fine.
answered Aug 16 at 11:17
pnzpnz
9164 silver badges17 bronze badges
I sure hope that QGIS provides functions for calculating "true" lengths without having to reproject manually.
– bugmenot123
Aug 16 at 13:19
Maybe try it...
– pnz
Aug 18 at 19:45
Yes, see QgsDistanceArea class
– ndawson
Aug 20 at 18:46
add a comment |
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2 Answers
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active
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2 Answers
2
active
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The $length expression function states that
The length calculated by this function respects both the current
project's ellipsoid setting and distance unit settings. For example,
if an ellipsoid has been set for the project then the calculated
length will be ellipsoidal, and if no ellipsoid is set then the
calculated length will be planimetric.
QgsGeometry.length on the other hand says
Returns the length of geometry using GEOS
Sadly this is not very helpful if you don't know what GEOS means. GEOS is the underlying geometry engine and it has no idea about coordinate systems, ellipsoids and all that spatial stuff. It is purely about geometry. This means QgsGeometry.length will consider all coordinate values of the geometry as flat cartesian coordinates. You might know they are geographic/ellipsoidal, GEOS does not.
If you are in a length-preserving projection with meaningful units (meters), the result will (almost) be the measurement as if done in the real world. But in all other cases, the results from this function are rather useless.
answered Aug 16 at 11:05
bugmenot123bugmenot123
4,55214 silver badges43 bronze badges
add a comment |
The $length expression function states that
The length calculated by this function respects both the current
project's ellipsoid setting and distance unit settings. For example,
if an ellipsoid has been set for the project then the calculated
length will be ellipsoidal, and if no ellipsoid is set then the
calculated length will be planimetric.
QgsGeometry.length on the other hand says
Returns the length of geometry using GEOS
Sadly this is not very helpful if you don't know what GEOS means. GEOS is the underlying geometry engine and it has no idea about coordinate systems, ellipsoids and all that spatial stuff. It is purely about geometry. This means QgsGeometry.length will consider all coordinate values of the geometry as flat cartesian coordinates. You might know they are geographic/ellipsoidal, GEOS does not.
If you are in a length-preserving projection with meaningful units (meters), the result will (almost) be the measurement as if done in the real world. But in all other cases, the results from this function are rather useless.
answered Aug 16 at 11:05
bugmenot123bugmenot123
4,55214 silver badges43 bronze badges
add a comment |
The $length expression function states that
The length calculated by this function respects both the current
project's ellipsoid setting and distance unit settings. For example,
if an ellipsoid has been set for the project then the calculated
length will be ellipsoidal, and if no ellipsoid is set then the
calculated length will be planimetric.
QgsGeometry.length on the other hand says
Returns the length of geometry using GEOS
Sadly this is not very helpful if you don't know what GEOS means. GEOS is the underlying geometry engine and it has no idea about coordinate systems, ellipsoids and all that spatial stuff. It is purely about geometry. This means QgsGeometry.length will consider all coordinate values of the geometry as flat cartesian coordinates. You might know they are geographic/ellipsoidal, GEOS does not.
If you are in a length-preserving projection with meaningful units (meters), the result will (almost) be the measurement as if done in the real world. But in all other cases, the results from this function are rather useless.
answered Aug 16 at 11:05
bugmenot123bugmenot123
4,55214 silver badges43 bronze badges
The $length expression function states that
The length calculated by this function respects both the current
project's ellipsoid setting and distance unit settings. For example,
if an ellipsoid has been set for the project then the calculated
length will be ellipsoidal, and if no ellipsoid is set then the
calculated length will be planimetric.
QgsGeometry.length on the other hand says
Returns the length of geometry using GEOS
Sadly this is not very helpful if you don't know what GEOS means. GEOS is the underlying geometry engine and it has no idea about coordinate systems, ellipsoids and all that spatial stuff. It is purely about geometry. This means QgsGeometry.length will consider all coordinate values of the geometry as flat cartesian coordinates. You might know they are geographic/ellipsoidal, GEOS does not.
If you are in a length-preserving projection with meaningful units (meters), the result will (almost) be the measurement as if done in the real world. But in all other cases, the results from this function are rather useless.
answered Aug 16 at 11:05
bugmenot123bugmenot123
4,55214 silver badges43 bronze badges
answered Aug 16 at 11:05
bugmenot123bugmenot123
4,55214 silver badges43 bronze badges
answered Aug 16 at 11:05
bugmenot123bugmenot123
4,55214 silver badges43 bronze badges
answered Aug 16 at 11:05
bugmenot123bugmenot123
4,55214 silver badges43 bronze badges
4,55214 silver badges43 bronze badges
add a comment |
add a comment |
Your layer's coordinate system is probably a Geographic one (meaning Ellipsoidal here), therefore the distance calculated in degree dimension. To solve this issue, first you have to a pick a proper Projected coordinate system, and save the layer in that coordinate system (Save as, change CRS).
You can find a proper Projected coordinate system at https://epsg.io, e.g. search for your country.
After the projection, the lenght() method's result will be fine.
answered Aug 16 at 11:17
pnzpnz
9164 silver badges17 bronze badges
I sure hope that QGIS provides functions for calculating "true" lengths without having to reproject manually.
– bugmenot123
Aug 16 at 13:19
Maybe try it...
– pnz
Aug 18 at 19:45
Yes, see QgsDistanceArea class
– ndawson
Aug 20 at 18:46
add a comment |
Your layer's coordinate system is probably a Geographic one (meaning Ellipsoidal here), therefore the distance calculated in degree dimension. To solve this issue, first you have to a pick a proper Projected coordinate system, and save the layer in that coordinate system (Save as, change CRS).
You can find a proper Projected coordinate system at https://epsg.io, e.g. search for your country.
After the projection, the lenght() method's result will be fine.
answered Aug 16 at 11:17
pnzpnz
9164 silver badges17 bronze badges
I sure hope that QGIS provides functions for calculating "true" lengths without having to reproject manually.
– bugmenot123
Aug 16 at 13:19
Maybe try it...
– pnz
Aug 18 at 19:45
Yes, see QgsDistanceArea class
– ndawson
Aug 20 at 18:46
add a comment |
Your layer's coordinate system is probably a Geographic one (meaning Ellipsoidal here), therefore the distance calculated in degree dimension. To solve this issue, first you have to a pick a proper Projected coordinate system, and save the layer in that coordinate system (Save as, change CRS).
You can find a proper Projected coordinate system at https://epsg.io, e.g. search for your country.
After the projection, the lenght() method's result will be fine.
answered Aug 16 at 11:17
pnzpnz
9164 silver badges17 bronze badges
Your layer's coordinate system is probably a Geographic one (meaning Ellipsoidal here), therefore the distance calculated in degree dimension. To solve this issue, first you have to a pick a proper Projected coordinate system, and save the layer in that coordinate system (Save as, change CRS).
You can find a proper Projected coordinate system at https://epsg.io, e.g. search for your country.
After the projection, the lenght() method's result will be fine.
answered Aug 16 at 11:17
pnzpnz
9164 silver badges17 bronze badges
answered Aug 16 at 11:17
pnzpnz
9164 silver badges17 bronze badges
answered Aug 16 at 11:17
pnzpnz
9164 silver badges17 bronze badges
answered Aug 16 at 11:17
pnzpnz
9164 silver badges17 bronze badges
9164 silver badges17 bronze badges
I sure hope that QGIS provides functions for calculating "true" lengths without having to reproject manually.
– bugmenot123
Aug 16 at 13:19
Maybe try it...
– pnz
Aug 18 at 19:45
Yes, see QgsDistanceArea class
– ndawson
Aug 20 at 18:46
add a comment |
I sure hope that QGIS provides functions for calculating "true" lengths without having to reproject manually.
– bugmenot123
Aug 16 at 13:19
Maybe try it...
– pnz
Aug 18 at 19:45
Yes, see QgsDistanceArea class
– ndawson
Aug 20 at 18:46
I sure hope that QGIS provides functions for calculating "true" lengths without having to reproject manually.
– bugmenot123
Aug 16 at 13:19
I sure hope that QGIS provides functions for calculating "true" lengths without having to reproject manually.
– bugmenot123
Aug 16 at 13:19
Maybe try it...
– pnz
Aug 18 at 19:45
Maybe try it...
– pnz
Aug 18 at 19:45
Yes, see QgsDistanceArea class
– ndawson
Aug 20 at 18:46
Yes, see QgsDistanceArea class
– ndawson
Aug 20 at 18:46
add a comment |
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