# How to Implement Elevation Profile Analysis in GeoLibre

> Learn to implement elevation profile analysis with GeoLibre. Sample terrain, compute statistics, and render interactive SVG charts. Export to CSV and SVG easily.

- Repository: [Open Geospatial Solutions/GeoLibre](https://github.com/opengeos/GeoLibre)
- Tags: how-to-guide
- Published: 2026-07-29

---

**GeoLibre's elevation profile feature enables users to draw a line on a MapLibre GL map, sample terrain elevations via the Open-Meteo API, compute distance statistics, and render interactive SVG charts that export to CSV and SVG formats.**

This implementation provides a complete end-to-end workflow for terrain analysis in geographic web applications. By combining client-side geometry processing with external elevation services, GeoLibre delivers a lightweight yet powerful tool for outdoor planning and terrain visualization. The following guide explains how to **implement elevation profile analysis** using the official GeoLibre source architecture.

## Architecture Overview

The elevation profile system in `opengeos/GeoLibre` follows a modular design split across five logical layers. Each layer handles specific responsibilities from user interaction to data persistence.

- **UI Control Layer** – [`ElevationProfileControl.ts`](https://github.com/opengeos/GeoLibre/blob/main/ElevationProfileControl.ts) manages the MapLibre GL control button, floating panel interface, and user interactions including line drawing, unit toggling, and state persistence.
- **Geometry & Statistics Layer** – [`geometry.ts`](https://github.com/opengeos/GeoLibre/blob/main/geometry.ts) provides pure-math utilities for resampling polylines, calculating haversine distances, and aggregating elevation statistics.
- **Data Fetching Layer** – [`client.ts`](https://github.com/opengeos/GeoLibre/blob/main/client.ts) interfaces with the Open-Meteo API endpoint (`https://api.open-meteo.com/v1/elevation`), handling timeouts and error classification.
- **Chart Rendering Layer** – [`profileChart.ts`](https://github.com/opengeos/GeoLibre/blob/main/profileChart.ts) generates SVG paths for line visualization and area fills, plus scaling functions for hover interactions.
- **Export Layer** – Helper methods in `ElevationProfileControl` serialize data to CSV and SVG formats, supporting both browser downloads and native file system integration via Tauri.

## Step-by-Step Implementation Workflow

### 1. Initialize the Map Control

The process begins when users activate the elevation profile button. The `_createMountainIcon` method renders the control interface, while the panel registers click handlers for the drawing mode. When activated, the system listens for map clicks via `_onMapClick`, storing vertices in the `_drawVertices` array until the user double-clicks or presses Enter to finish.

### 2. Resample the Line Geometry

Once drawing completes, the `_profileLine` method calls `resampleLine` from [`packages/plugins/src/plugins/elevation-profile/elevation/geometry.ts`](https://github.com/opengeos/GeoLibre/blob/main/packages/plugins/src/plugins/elevation-profile/elevation/geometry.ts). This function interpolates the user-drawn polyline into uniformly spaced segments, producing `sampled.coords` and `sampled.distances` arrays. The resampling respects the `MAX_POINTS_PER_REQUEST` constant (set to **100**), ensuring API compliance while maintaining geographic accuracy.

### 3. Fetch Elevation Data

The `fetchElevations` function in [`client.ts`](https://github.com/opengeos/GeoLibre/blob/main/client.ts) constructs a query string from the resampled coordinates and sends a GET request to the Open-Meteo elevation endpoint. The implementation includes:
- Timeout handling for network resilience
- Response validation ensuring coordinate/elevation array length parity
- Error wrapping via the custom `ElevationFetchError` class for debugging

### 4. Calculate Terrain Statistics

With elevation data returned, `computeStats` (in [`geometry.ts`](https://github.com/opengeos/GeoLibre/blob/main/geometry.ts)) iterates through the height array to derive:
- **Minimum** and **maximum** elevation values
- Cumulative **ascent** (gain) and **descent** (loss)
- **Total distance** along the profile line

These metrics update the statistics panel in real-time.

### 5. Build Chart Geometry

The `buildChartGeometry` function in [`profileChart.ts`](https://github.com/opengeos/GeoLibre/blob/main/profileChart.ts) transforms the distance/elevation pairs into SVG-ready data structures:
- `linePath` – SVG path commands for the profile line
- `areaPath` – Closed path for the filled area beneath the line
- `xScale` / `yScale` – Linear mapping functions between data values and pixel coordinates
- `indexForX` – Binary search utility for hover position lookup

### 6. Render Interactive Visualization

The `_renderChart` method constructs the SVG DOM element, attaches mousemove handlers for hover tracking, and updates the read-out box with distance and elevation values at the cursor position. The chart automatically adapts to the `panelWidth` configuration parameter.

### 7. Export Results

Users export data through two pathways:
- **CSV Export**: `_exportCsv` calls `profileToCsv` (in [`export/csv.ts`](https://github.com/opengeos/GeoLibre/blob/main/export/csv.ts)) to format coordinates and elevations as comma-separated values
- **SVG Export**: `_exportSvg` inlines computed CSS styles and adds a background rectangle via `_buildExportSvg` for print-ready graphics

Both methods use the injected `ExportTextFile` callback when running in Tauri environments, falling back to browser Blob downloads otherwise.

### 8. Persist State

The `getState` and `setState` methods enable serialization of the profile state (`collapsed`, `unitSystem`, `line`), allowing GeoLibre to restore profiles from project files or deep-link URLs via the `loadLine` method.

## Key Source Files and Their Functions

| File | Key Function | Purpose |
|------|--------------|---------|
| [`core/ElevationProfileControl.ts`](https://github.com/opengeos/GeoLibre/blob/main/core/ElevationProfileControl.ts) | `loadLine()`, `_exportCsv()`, `_exportSvg()` | Main MapLibre control class managing UI lifecycle, drawing logic, and exports |
| [`elevation/geometry.ts`](https://github.com/opengeos/GeoLibre/blob/main/elevation/geometry.ts) | `resampleLine()`, `computeStats()` | Pure geometric calculations including haversine distance and elevation aggregation |
| [`elevation/client.ts`](https://github.com/opengeos/GeoLibre/blob/main/elevation/client.ts) | `fetchElevations()` | Open-Meteo API wrapper with error handling and timeout management |
| [`chart/profileChart.ts`](https://github.com/opengeos/GeoLibre/blob/main/chart/profileChart.ts) | `buildChartGeometry()` | SVG path generation and scaling function creation |
| [`export/csv.ts`](https://github.com/opengeos/GeoLibre/blob/main/export/csv.ts) | `profileToCsv()` | Data serialization for spreadsheet export |
| [`utils/deep-link.ts`](https://github.com/opengeos/GeoLibre/blob/main/utils/deep-link.ts) | URL parameter parsing | Helper for restoring profiles from query strings |

## Practical Implementation Examples

### Adding the Control to Your Map

Instantiate the control with your preferred units and sample limits:

```typescript
import { ElevationProfileControl } from
  'packages/plugins/src/plugins/elevation-profile/core/ElevationProfileControl';

const elevationControl = new ElevationProfileControl({
  title: 'Elevation Profile',
  unitSystem: 'metric',   // or 'imperial'
  panelWidth: 340,
  maxSamples: 80,        // Must be < MAX_POINTS_PER_REQUEST (100)
});

map.addControl(elevationControl, 'top-right');

```

### Loading a Line Programmatically

Restore profiles from saved coordinates or deep links:

```typescript
const line = [
  [-122.45, 37.78],
  [-122.44, 37.79],
  [-122.43, 37.80],
];

await elevationControl.loadLine(line);   // Draws line, fetches data, fits bounds

```

### Triggering Exports

Call export methods directly from UI event handlers:

```typescript
// Download as CSV
elevationControl._exportCsv();

// Download as SVG
elevationControl._exportSvg();

```

### Switching Unit Systems

Cycle between measurement systems programmatically:

```typescript
elevationControl._cycleUnits();   // Toggles metric ↔ imperial, updates labels

```

## Error Handling and Performance Considerations

The implementation includes robust error boundaries. When `fetchElevations` encounters network timeouts, HTTP failures, or payload mismatches, it throws `ElevationFetchError` with descriptive context rather than failing silently. 

For performance, the `maxSamples` configuration parameter (defaulting to 100) prevents API rate limiting while maintaining visual fidelity. The resampling algorithm in [`geometry.ts`](https://github.com/opengeos/GeoLibre/blob/main/geometry.ts) uses spherical haversine calculations for accurate distance measurement across large geographic spans.

## Summary

- **GeoLibre** implements elevation profile analysis through a five-layer architecture separating UI, geometry, network, rendering, and export concerns.
- The **Open-Meteo API** provides free elevation data with a 100-point limit per request, handled transparently by the resampling logic in [`geometry.ts`](https://github.com/opengeos/GeoLibre/blob/main/geometry.ts).
- **Interactive SVG charts** are generated serverlessly via [`profileChart.ts`](https://github.com/opengeos/GeoLibre/blob/main/profileChart.ts), supporting hover interactions and responsive scaling.
- **State persistence** methods enable deep-linking and project save/restore functionality across sessions.
- **Export utilities** support both CSV (data analysis) and SVG (visualization) workflows with automatic environment detection.

## Frequently Asked Questions

### How does GeoLibre handle the Open-Meteo API's 100-point limit?

The `resampleLine` function in [`geometry.ts`](https://github.com/opengeos/GeoLibre/blob/main/geometry.ts) automatically interpolates user-drawn lines to meet the `MAX_POINTS_PER_REQUEST` constant (100 points). You can configure a lower limit via the `maxSamples` option in the control constructor to reduce API calls, though this reduces profile resolution.

### What coordinate format does the elevation profile expect?

All internal functions expect arrays of `[longitude, latitude]` number pairs (EPSG:4326). The `loadLine()` method validates that input arrays contain at least two points before processing, throwing descriptive errors for malformed input.

### Can I customize the chart appearance?

The `buildChartGeometry` function in [`profileChart.ts`](https://github.com/opengeos/GeoLibre/blob/main/profileChart.ts) returns raw SVG path data and scaling functions, allowing you to override the `_renderChart` method in [`ElevationProfileControl.ts`](https://github.com/opengeos/GeoLibre/blob/main/ElevationProfileControl.ts) to inject custom CSS classes or modify the SVG structure before DOM insertion.

### Does the elevation profile work offline?

No, the current implementation requires internet connectivity to reach `https://api.open-meteo.com/v1/elevation`. However, the geometry calculation and chart rendering modules are pure JavaScript functions that operate offline once elevation data is cached or provided programmatically via `loadLine()`.