Calculating And Adding Latitude/Longitude Fields To Layers In Qgis

Working with geographic data in QGIS often involves using latitude and longitude coordinates to locate features on a map. By adding dedicated latitude and longitude columns to attribute tables, you can enable powerful spatial analysis and mapping capabilities.

Prerequisites: Installing Required Plugins

Certain plugins extend QGIS functionality for calculating and storing latitude/longitude data for layers:

• Delimited Text – Imports/exports tabular data with delimiters
• MMQGIS – Contains handy tools for coordinate transformations
• OpenLayers – Displays basemaps from web map services

Install these plugins via the Plugin Manager before starting to work with latitude/longitude data in QGIS.

Viewing Latitude/Longitude Data in QGIS

Once coordinate data is compiled or imported into QGIS, enabling on-the-fly projection transforms layer data to the project’s defined coordinate reference system (CRS). This allows visualizing point, line, and polygon geometries accurately on a basemap.

Basemaps provide critical geographic context – the OpenLayers plugin facilitates adding basemaps like OpenStreetMap.

Example Workflow

1. Import .csv file containing site ID, name, lat, long fields
2. Enable OTF CRS transformation for layer to WGS 84 (EPSG:4326)
3. Add OpenStreetMap via OpenLayers plugin
4. Verify site points overlay properly with basemap locations

Calculating Latitude/Longitude Fields for Point Layers

QGIS possesses tools to derive or calculate latitude/longitude coordinates for point geometry layers:

• Points to Paths – encode coordinates in polyline paths
• Export/Add Geometry Columns – exposes coordinates
• Field Calculator with geom_to_wkt() function
• MMQGIS Coordinate Export/Import – batch conversion capabilities

These approaches output new data, facilitating adding dedicated latitude/longitude attribute fields – critical for spatial analysis.

Example Workflow

1. Input point layer containing site locations
2. Use MMQGIS Export Geometry to Points to generate lat/long values
3. Join output table to original layer
4. Calculate two new float type fields called “Latitude” and “Longitude”
5. Populate fields with exported coordinate values

Adding Latitude/Longitude Columns to Attribute Tables

Once calculated or compiled, incorporate latitude/longitude data into point layer attribute tables via:

• Creating new double or float type fields
• Field calculator to populate values
• Linking tables from delimited text imports
• Joining other attribute/coordinate exports

This establishes dedicated storage for coordinate values, cementing the link between attribute data and geometry locations.

Example Workflow

1. Input point file with 30 site locations as layer geometry
2. Export coordinates with Points to Paths tool
3. Open attribute table, toggle editing mode
4. Add two float type fields: Latitude, Longitude
5. Use field calculator with Join layer to populate fields
6. Save edits and disable editing mode

Populating Latitude/Longitude Fields

With target fields added, various methods to populate latitude/longitude data values:

• Field calculator expressions referencing functions
• Join attributes from coordinate export files
• Links to delimited text tables with coordinates
• Upload results from GPS units
• Digitizing new points with auto-population

Checking populated values via the Identify tool confirms accurate coordinate storage.

Example Workflow

1. Input layer has two float fields Latitude and Longitude
2. Use Points to Paths, export output to delimited text
3. In main layer attribute table, open field calculator
4. Write calculated expressions to join exported fields
5. Apply and save edits, view updates via Identify tool

Setting Coordinate Reference Systems

QGIS layers can use different coordinate systems. Accurately displaying and analyzing latitude/longitude data requires properly defined projections:

• OTF Reprojection on the fly transforms layer data
• Assign Project CRS to standardize analyses
• Define Layer CRS if OTF unable to detect
• Transformations utilize official EPSG definitions

Modifying layer and project properties facilitates correct CRS use with latitude/longitude values in QGIS.

Example Workflow

1. New project, unprojected coordinate system by default
2. Import country-level point dataset with WGS 84 values
3. Assign project CRS as WGS 84 (EPSG:4326)
4. Enable OTF for layers to visualize properly
5. Verify coordinates via identify tool

Using Latitude/Longitude Data in Analysis

With dedicated numeric fields storing coordinate pairs, QGIS unlocks robust spatial analysis tools using latitude/longitude values:

• Layer filtering, selection – proximity, thresholds
• Geoprocessing tools – buffer, clip, intersect
• Geometry related calculations
• Field aggregates using $x, $y expressions
• Label, diagram, map element positioning

Many core QGIS functions utilize defined coordinate data stored in layer attribute tables.

Example Workflow

1. Input point site location data with lat/long fields
2. Select sites within defined latitude threshold
3. Buffer selected set 300 meters to define regions
4. Aggregate total site counts per region
5. Join aggregated data back to display on map

Example: Creating Heat Maps from Latitude/Longitude

An applied use case for structured latitude/longitude data is generating insightful heat maps in QGIS:

1. Collect case data with accurate geographic coordinates
2. Import to QGIS, transform layer to a projected CRS
3. Use Point Sampling Tool to rasterize vector into gridded format
4. Style raster heights as color ramps to reveal hot spots
5. Apply transparency to overlay on basemaps

This workflow transforms site coordinates into visual cluster analysis – quickly revealing trends through the power of latitude/longitude data.

Troubleshooting Common Latitude/Longitude Issues

Working with geographic coordinates can pose certain challenges – strategies to overcome include:

• Confirm coordinate reference systems – assign projection for layers/project
• Check for digitizing errors if points fail to align with basemaps
• Inspect data import processes to verify proper delimiters
• Use Define Current Projection tool to overwrite bad CRS info
• Employ MMQGIS transforms to calculate missing/accurate coordinates

Paying attention to projection definitions and carefully validating locations will produce quality latitude/longitude data for QGIS.