mcp-server-gis-helper

An MCP server that lets a user draw GIS geometry on an interactive map and returns clean, validated GeoJSON — ready to hand off to other GIS MCP servers such as FEMA NFHL flood screening or USGS National Map hydrography.

It solves the "how do I generate a complex shape and pass it to the geometry-based tools?" problem: instead of hand-typing coordinates, the user sketches a polygon, line, or point on a Leaflet map and gets back a compact GeoJSON string.

This server does not query flood/waterway services itself. It only produces and validates geometry; the agent passes that geometry to the downstream tools.

Tools

Tool	Purpose
gis_open_map	Interactive entry point. Returns a Prefab MCP App with a link to a browser drawing map and a "Load drawn geometry" button. Optional lat/lon/zoom center the map.
gis_get_drawn_geometry	Retrieves the shape the user drew and saved on the map for a given session_id. Returns a compact GeoJSON string.
gis_validate_geometry	Validates/normalizes an arbitrary GeoJSON string you already have (no map needed). Closes polygon rings, checks WGS84 ranges, unwraps Feature/FeatureCollection.

All geometry is WGS84 decimal degrees in GeoJSON [lon, lat] order. Supported types: Point, MultiPoint, LineString, MultiLineString, Polygon, MultiPolygon, plus the BoundingBox shorthand ({"type": "BoundingBox", "bbox": [minLon, minLat, maxLon, maxLat]}) — exactly what the FEMA/USGS tools accept.

How it works (the map bridge)

Prefab's Embed renders a sandboxed iframe whose content cannot write back into Prefab state, so the drawing map cannot live purely inside the MCP app. Instead a small companion HTTP "map bridge" (Starlette) serves the Leaflet + Leaflet.draw page:

1. gis_open_map creates a draw session and returns a Prefab app with a link to <public_url>/map?session=<id>.
2. The user opens that link in a browser, draws one shape, and clicks Save. The page POSTs the GeoJSON to <public_url>/session/<id>/geometry, where it's validated and stored in an in-memory session store.
3. gis_get_drawn_geometry reads the stored geometry out of the shared store and returns it as a compact GeoJSON string.
4. The agent passes that string to nfhl_screen_flood_zone, hydro_find_waterways, etc.

Transports (mirrors the sibling GIS servers)

• stdio (local MCP clients like Claude Desktop/Code): MCP runs over stdio and the map bridge is started on a localhost daemon thread (default 127.0.0.1:8765).
• HTTP (deployed / containerized): when PORT (or DATABRICKS_APP_PORT) is set, MCP is served at /mcp and the bridge routes (/map, /session/{id}/geometry, /health) are mounted onto the same port.

Configuration (all optional)

Env var	Default	Purpose
GIS_HELPER_MAP_HOST	127.0.0.1	Bridge bind host (stdio mode)
GIS_HELPER_MAP_PORT	8765	Bridge port (stdio mode)
GIS_HELPER_PUBLIC_URL	http://<host>:<port>	Origin the browser uses to reach the bridge (set behind a proxy/container)
GIS_HELPER_TILE_URL	OpenStreetMap	Leaflet basemap tile URL template
GIS_HELPER_TILE_ATTRIBUTION	OSM	Tile attribution string
PORT / DATABRICKS_APP_PORT	—	If set, selects HTTP transport for MCP

Copy .env.example to .env for local overrides.

Running

uv sync

# stdio (local MCP client)
uv run python main.py

# HTTP (deployed)
PORT=8080 uv run python main.py

Example workflow

User:  I want to screen a project area for FEMA flood zones.
Agent: (calls gis_open_map centered on the site)
User:  (draws a polygon on the map, clicks Save, clicks "Load drawn geometry")
Agent: (gis_get_drawn_geometry -> geometry string)
Agent: (passes geometry to nfhl_screen_flood_zone)

If the user already has coordinates, skip the map and use gis_validate_geometry to clean them up before querying downstream.

Project layout

src/gis_helper_mcp/
├── app.py            # FastMCP init, instructions, transport, bridge startup
├── models.py         # dataclasses (DrawSession, GeometrySummary) + constants
├── utils.py          # GeoJSON validation/normalization, summaries, session store, config
├── bridge.py         # Starlette map-bridge routes + runners
├── static/
│   └── map.html      # Leaflet + Leaflet.draw drawing page (CDN assets)
└── tools/
    ├── __init__.py           # register_tools(mcp)
    ├── open_map.py           # gis_open_map (Prefab app)
    ├── get_drawn_geometry.py # gis_get_drawn_geometry
    └── validate_geometry.py  # gis_validate_geometry