Turn a Spreadsheet Into a Spatial Picture
A table of numbers is hard to read at a glance — a map is not. This tool takes any Excel spreadsheet with a latitude column, a longitude column, and any numeric value column (population, sales, temperature, survey scores, anything), and interpolates a smooth, continuous heat map showing how that value varies across the region, even between the points you actually measured.
How IDW Interpolation Works
The tool uses Inverse Distance Weighting (IDW), a well-established spatial interpolation method: to estimate the value at any location, it takes a weighted average of every known data point, giving more influence to points that are closer and less to points further away.
The Power Setting
The Power slider controls how quickly that influence fades with distance. A low power (close to 1) blends values smoothly over a wide area; a high power (close to 4) makes each data point dominate its immediate surroundings more sharply, producing more localised, less-blended contours.
Resolution vs. Performance
Resolution controls the size of the grid used to compute the interpolation — Low is fastest, Very High produces the smoothest gradient but takes longer to render, especially with many data points. Start with Medium and increase it once you're happy with your column selection and colours.
Clipped to Your Data's Shape
The coloured fill is clipped to the convex hull of your points — the tightest boundary that encloses all of them — rather than a rectangle. A thin dashed outline marks this boundary on the map, so the heat map never bleeds into empty corners far from any point you actually have data for.
Preparing Your Excel or CSV File
Your spreadsheet needs three things, in any column order:
- A latitude column: decimal degrees, e.g. 40.7128.
- A longitude column: decimal degrees, e.g. -74.0060.
- A numeric value column: whatever you want to visualise — population, sales, temperature, a survey score, anything numeric.
Upload the file, confirm (or correct) the three column selections in the sidebar, then click "Generate Heatmap". The map zooms to fit your data and renders the interpolated surface.
Styling the Heat Map
Three controls in the sidebar shape how the heat map reads:
- Colour Gradient: set the Low, Mid and High colours — the default green-to-red mimics a classic risk/intensity scale, but any three colours work.
- Opacity: from a subtle 20% wash to a fully solid 100% fill.
- Power & Resolution: control how sharply values blend between points, and how fine the rendering grid is.
Every change re-renders the heat map instantly using your already-loaded data — no need to re-upload or regenerate.
Exporting Your Heat Map
Click "Download Heatmap (PNG)" to save an image combining the basemap and the heat map overlay together, ready to drop into a report or presentation. If the basemap tiles can't be captured due to the tile server's security policy, the tool automatically falls back to exporting just the heat map layer on its own.
Step-by-Step Guide
Common Use Cases
Frequently Asked Questions
Related Tools
Map tiles © OpenStreetMap contributors and CARTO, Esri (satellite). Excel parsing powered by SheetJS. Map rendering by Leaflet. All parsing and interpolation is computed locally in your browser — no data is uploaded to any server.