Questions tagged [expression]
Expressions are SQL-like statements that are used to evaluate dynamic properties.
172
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Use QGIS expressions to create geometries: difference between geometry generator and geometry by expression
QGIS has a powerful expression machine that can be used (between other things) to create new geometries, based on existing layers. Basically, there are two options: Geometry generator (introduced in ...
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Projection transformation of a shapefile
I am using QGIS 3.18.1.
I am working on a map around Alaska. I added a world basemap, then added DEM and finally drew a few polygon shapefiles on top.
I then tried to find a projection more ...
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6
votes
1
answer
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Buffer is too small (OSM Data, EPSG 3857)
I buffered some lines with 100 m in QGIS. When I measured it turns out instead of 100 m its 62. I understand the issue could relate to the CRS. I used EPSG 3857 because the OSM map in the background ...
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6
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3
answers
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Calculating interior angles of polygons or lines in QGIS
The problem
In QGIS, it's easy to calculate areas of polygons or lengths of lines using QGIS expressions with area ($geometry) / $area or length($geoemtry) / $length respectively. However, there is no ...
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votes
4
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Creating perpendicular lines on line using QGIS
In QGIS I want to create perpendicular Lines on another Line (River). The Lines should be created on points on the Line and should have a length of 40 Meters. I want to create the red lines in the ...
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13
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3
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Difference between quotation marks ('single' vs "double") in QGIS
I am confused concerning the quotation marks syntax ('single' vs "double") in QGIS.
I know something about the quotation marks and the way they are applied in Python scripting (i.e. comments,...
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3
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QGIS expression with overlay-fuction: filter-condition based on comparison of attributes from two layers
In QGIS 3.18, I have two point layers: layer_1 with attribute id_1 and layer_2 with id_2. I want to use the overlay_nearest function: overlay_nearest(layer[,expression][,filter][,limit=1][,...
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1
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QGIS query syntax differs between attribute table and feature filter
Using QGIS 2.16.2, I can successfully select records from a shapefile attribute table using the following expression:
left("start_time", 10) = '2015-08-01'
Please note that start_time is a string ...
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10
votes
1
answer
865
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Re-locating points randomly at certain distance in QGIS
I have GPS coordinates which identify aid projects in Africa. Now, I want to re-locate these projects, such that each project is re-located by 100 km in a random direction from its original location.
(...
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8
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2
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Achieving Super or Subscript graticule labels in QGIS Composer windows?
It is quite common for paper topographic maps to label the map graticule with ordinate values in a mixture of superscript and standard digits, eg. ⁴123⁰⁰⁰ to emphasise the significant digits.
The ...
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7
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1
answer
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Refering to the input-layer of a QGIS model in field calculator expression
The setting
In a QGIS model, I have an input layer polygon from which a grid is created.
On this grid-layer, a new attribute field should be created based on an expression that refers to the input ...
- 71.2k
4
votes
2
answers
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Colouring points in the same colour as their nearest point/polygon from another layer in QGIS
I got two files, a multi-polygon file with polygons and points (File 1) and a file with points only (File 2).
Our task is to fill the color of the points from File 2 with the respectively nearest ...
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28
votes
2
answers
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What is the difference between != NULL and IS NOT NULL in QGIS filter expressions?
I have a shape layer with an attribute containing NULL values to which I would like to apply a filter for values different from NULL.
Using the available GUI for such query construction, one would ...
- 14.6k
7
votes
1
answer
3k
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Labeling only one duplicate attribute without editing original data in QGIS
I have several points (e.g. 4 bus stop positions) in one area. They all belong to the same station, so they all have the same "name" attribute. To avoid a mess of labels showing all the same ...
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6
votes
2
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Getting lat/long for centroids using QGIS
I have a series of polygons (lakes) in Kansas, Oklahoma and a few in Texas. I created centroids for each lake, which I needed for distance measurements.
These centroids have derived lat/long ...
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2
votes
1
answer
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Assigning specific feature in expression builder
I have a question regarding "Making tiny objects visible for their location when zooming out" in QGIS: Making tiny objects visible for their location when zooming out
or more specifically ...
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15
votes
4
answers
4k
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Rendering dashed lines between adjacent polygons correctly in QGIS
I have adjacent polygons that I want to be marked out with dashed or dotted lines. This sometimes works but there appears to be a problem where the pattern isn't always synchronised on the shared ...
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11
votes
2
answers
11k
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How to get attribute values of another layer with QGIS expressions
The question: How is it possible to get the value of an attribute in another layer?
What is clear: When using QGIS expressions on a layer, it is possible to relate to features of another layer with ...
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10
votes
2
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QGIS Multiple CASE WHEN/THEN Statements for Expression Based Labels
I am trying to label a single point with multiple labels, based on the attributes for that point. For example, if one of the attributes is equal to 1, I want to label it with a specific text string; ...
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10
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0
answers
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Performance-limits of QGIS expressions
I use quite often QGIS expressions and I am wondering how performant they are compared with other possibilities. In many cases, the same problem can be resolved using different approaches like using ...
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8
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1
answer
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How to build a query/expression to select specific points within a buffer? (in QGIS)
I have a point layer in QGIS. Each point represents a tree and each tree has information about its height as an attribute.
I would like to build a query/expression to select all those trees that have ...
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7
votes
2
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Getting currentfeature's color in Expression Builder of QGIS?
In QGIS Expression builder. I want to obtain the color code of the current feature.
I mean:
My layer contains lines, each one is painted with a different color. Now I want to assign that color to ...
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7
votes
2
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Extracting text within parenthesis in a QGIS attribute table
I have a shapefile of data for Japan, and I need to extract the English character names in my "name" field in order to label the layer. Much of the data is all in Japanese, but some of the names have ...
- 301
6
votes
2
answers
645
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Saving frequently used expressions in QGIS
Is there any way to save frequently used expressions in the selection or field calculator dialogs? Obviously I can cut and paste them into a file where I might save them, but I mean within the context ...
- 5,963
5
votes
1
answer
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Using round function in QGIS expression?
I created a grid of rectangular polygons with the Create Grid function, from the Vector Creation group of the Processing Toolbox in QGIS 3.4.2.
The rectangles measure half a degree of longitude by ...
- 14.3k
4
votes
1
answer
781
views
QGIS vector layer interactions: calculate percentage of polygon intersection area
I am new to QGIS and have two vector layers (the green one, displayed in graduated symbology, and separately the orange layer - see image pasted below). I want to create a new field in the attribute ...
- 157
3
votes
1
answer
1k
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QGIS Geometry Generator: make_line once per grouped geometries
I am using a Geometry Generator symbol layer to draw lines between grouped points. I am grouping points based on two fields letter and number, which I concatenate to make the groups (GeoPackage ...
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3
votes
1
answer
469
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Can the ArcGIS solution for uneven buffering be achieved in QGIS? version 2 - uneven tree canopies
I want to create tree canopies with uneven canopy extents.
A previous solution is below:
Representing uneven tree canopy spread around point in QGIS
A solution for uneven buffering has been provided ...
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2
votes
3
answers
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Extract by expression - N first
How can I "extract by expression" the N first (10 highest values of one column, for example)
I need it for the graphical modeler.
20
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3
answers
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Learning to use expressions in QGIS?
Is there any resource recommendations in general for getting past the basics of QGIS and going into the deep dark secrets?
Specifically in relation to expression coding for labeling, queries, etc.
13
votes
3
answers
18k
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Extracting part of string from field in QGIS Field Calculator?
I want to create a new field in my attribute table. My goal: returning all digits between the / and _ characters of the field named "location" by applying the next expression:
regexp_substr( ...
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11
votes
2
answers
1k
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Applying labelling with obstacles in QGIS
I'm working with QGIS 3.16 Hannover and I'm quite confused with the option Features act as obstacles.
According to QGIS documentation (13. The Style Library » 13.3. Setting a label » 13.3.2.3.4.4. ...
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votes
2
answers
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Is there an escape option for single quotes & apostrophes in QGIS?
I'm trying to use the Data Defined symbology in QGIS ver 2.2. Some of my data has single quotes/apostrophes.
Since the string must use single quotes, is there an escape option I can use for the field ...
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6
votes
3
answers
955
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Calculating distance between multiple points within buffer using QGIS
I am very new to QGIS.
Attached is an image of my QGIS project so far. In this, you have the pinkish points of the "Point_centers" layer and green points labeled "Point_trees" ...
5
votes
1
answer
2k
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referencing to features geometry in an expression
I have a layerA with roads and a layerB with one shortest path.
If I put an aggregate function into a virtual field in layerB to calculate the sum of fieldAA where the shortest path contains features ...
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5
votes
2
answers
207
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Counting number of points in one layer within fixed distance to points in another layer in QGIS
In QGIS 3.22.7 I want to count the number of points on point 'layer B' within 60m of each point on point 'layer A' and add a column to the data on point 'layer A'. What is wrong with the following ...
5
votes
1
answer
2k
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Use field calculator to check for intersection
Ultimate goal
I plan to retrieve attributes from layers using the field calculator where they intersect with a geometry.
Current step
Checking whether the current $geometry intersects with the layer ...
- 16.3k
5
votes
2
answers
443
views
Creating line between point layer and line layer in QGIS
My goal is to create a line layer that is derived from the distance between a point layer and a line layer. An example could be using a line layer representing a road and a point layer representing a ...
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5
votes
1
answer
280
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Hiding layer for N seconds in QGIS: animate symbols with custom intervals
This is a follow-up to Making geometry blink in QGIS, for which I couldn't find any answer online.
It is relatively simple, using the Time Manager plugin or the Temporal Controller native tool in QGIS,...
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votes
3
answers
643
views
Define a unique ID (serial number) based on values in a field
I'm trying to generate a unique serial number for a dataset I have, for example, something like this:
As shown, I would like to have a serial field, called SERIAL in this case, that increments based ...
4
votes
2
answers
9k
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How to convert decimal point to comma in QGIS?
I am working with data in Colombia where decimal points are written as commas. I would love to create a new column in QGIS that would convert all decimal points (periods) to commas for the purpose of ...
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4
votes
2
answers
2k
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Optimizing QgsFeatureRequest with filter expression
I have a point PostGIS layer (my_layer) which has more than 66000 features. I am getting the features I want but it takes too long. My code is:
resultList = []
req = QgsFeatureRequest()....
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4
votes
1
answer
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Summing up the area on a polygon by a certain attribute result
I wish to know if there's a tool on QGIS equivalent to the Excel's "SUMIF".
I have a shapefile of a land use and cover that's classified by the attribute with the name of the classes, for example: "...
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4
votes
1
answer
2k
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Extracting GeoTIFF colours and adding to equal geometry shapefile
I have the following map in .png-format that I would like to replicate, but I cannot obtain the original data.
The map shows the current water supply vulnerability for local municipalities in South ...
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4
votes
0
answers
2k
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QGIS expression for label text to display vertex coordinates of polygons
This question is inspired by Displaying vertex coordinates of a polygon or line without creating a new layer and especially by the answer by @JGH: https://gis.stackexchange.com/a/381130/88814
The ...
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4
votes
1
answer
812
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Create an inscribed square inside a polygon
I have polygons of the administrative boundaries and want to create a square polygon inside of them, which will have the maximum area and will be fully covered by these boundaries. In other words: how ...
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4
votes
2
answers
842
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Numbering points per polygon group with QGIS expressions
In QGIS, I have a point and a polygon layer. I want to number the points ascending from left to right (from smallest to highest x-coordinate value - West to East). However, counting should be made ...
- 71.2k
4
votes
1
answer
667
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Calculating the percentage of overlapping areas from different layers in QGIS
I work with QGIS 3.16 Hannover and I have three polygon layers 'A', 'B', 'C' with multiple geometries in each.
In the image, layer 'A' has the bold polygons and, for each of them (meaning within each ...
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3
votes
1
answer
930
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Using QGIS expression with overlay_nearest() function to create line connecting point to two nearest points of other layer
I have in QGIS ver 3.22 two point layers named:
source_p
target_p
I want to point automatically with a line from the source_p layer to the two nearest points on the target_p layer.
Manually, I was ...
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3
votes
2
answers
2k
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Limiting available values in dropdown field of QGIS form according to value selected in previous field?
I'm building a rather complex entry form for a layer stored in a PostGIS database. I am following this usage scenario:
On several occasions I have dropdown fields which lets the user select a type of ...
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