Coordinate Lookup

Coordinate Lookup

Overview

Find accurate GPS coordinates for locations using OpenStreetMap's Nominatim geocoding service. This skill provides knowledge for querying Nominatim API, handling multiple search strategies, and selecting the most relevant results.

When to Use This Skill

Load this skill when:

• Finding GPS coordinates for a location name
• Geocoding addresses or place names
• Resolving ambiguous location queries
• Validating coordinate accuracy
• Troubleshooting failed coordinate lookups

Core Concepts

Nominatim API

Nominatim is OpenStreetMap's free geocoding service that converts place names and addresses into geographic coordinates.

Key features:

• Free to use (no API key required)
• Global coverage from OpenStreetMap data
• Multiple search strategies (place name, address, structured)
• Returns multiple results ranked by relevance
• Includes bounding boxes and administrative boundaries

Base URL: https://nominatim.openstreetmap.org

Rate limit: 1 request per second (enforced)

Search Endpoint

Basic search:

GET /search?q={query}&format=json&limit=5

Parameters:

• q - Search query (location name, address, etc.)
• format - Response format (json, jsonv2, xml)
• limit - Maximum results (default 10, max 50)
• countrycodes - Restrict to countries (ISO 3166-1 alpha-2, comma-separated)
• addressdetails - Include address breakdown (0 or 1)

Example:

curl "https://nominatim.openstreetmap.org/search?q=Old+Man+of+Storr&format=json&limit=5"

Response Format

[
  {
    "place_id": 12345,
    "lat": "57.5062935",
    "lon": "-6.180885",
    "display_name": "Old Man of Storr, Isle of Skye, Highland, Scotland, UK",
    "type": "peak",
    "importance": 0.62,
    "boundingbox": ["57.5062935", "57.5062935", "-6.180885", "-6.180885"]
  }
]

Key fields:

• lat - Latitude (string, convert to number)
• lon - Longitude (string, convert to number)
• display_name - Full location description
• type - OSM feature type (peak, city, restaurant, etc.)
• importance - Relevance score (0-1, higher = more important)
• boundingbox - [min_lat, max_lat, min_lon, max_lon]

Search Strategies

To find the most accurate coordinates, try multiple search strategies in order:

1. Exact Name Search

Start with the exact location name provided.

curl "https://nominatim.openstreetmap.org/search?q=University+of+Bristol+Botanic+Gardens&format=json&limit=3"

Best for: Well-known landmarks, specific venues

2. Name + Country/Region

Add geographic context if exact search fails or returns too many results.

curl "https://nominatim.openstreetmap.org/search?q=Dishoom+Edinburgh&countrycodes=gb&format=json&limit=3"

Best for: Common names, chain locations

3. Structured Search

Use structured query parameters for addresses.

GET /search?street={street}&city={city}&country={country}&format=json

Best for: Full addresses, specific properties

4. Reverse Search Variations

Try alternative name formats or common misspellings.

# Try without special characters
curl "https://nominatim.openstreetmap.org/search?q=Old+Man+Storr&format=json"

# Try alternative names
curl "https://nominatim.openstreetmap.org/search?q=The+Old+Man+of+Storr&format=json"

Selecting Best Result

When multiple results returned, select based on:

1. Type relevance: Match expected type (restaurant, peak, park, etc.)
2. Importance score: Higher is generally more relevant
3. Display name: Should match expected location/region
4. Bounding box: Smaller box = more specific location

Example logic:

# Sort by importance (descending)
results.sort(key=lambda x: float(x['importance']), reverse=True)

# Filter by expected type if known
if location_type == "restaurant":
    results = [r for r in results if r['type'] in ['restaurant', 'cafe', 'bar']]

# Take first (most relevant)
best_result = results[0]
coordinates = [float(best_result['lat']), float(best_result['lon'])]

Error Handling

No Results Found

Causes:

• Misspelled location name
• Very obscure or new location
• Location not in OpenStreetMap database

Solutions:

1. Try alternative name formats
2. Add geographic context (country, region)
3. Search for nearby landmark instead
4. Leave coordinates empty (graceful degradation)

Rate Limit Exceeded

Error: HTTP 429 or "Too Many Requests"

Solution:

• Wait 1 second between requests
• Use time.sleep(1) in scripts
• Respect Nominatim usage policy

Ambiguous Results

Issue: Multiple equally relevant results

Solutions:

1. Check display_name for region match
2. Use countrycodes parameter to filter
3. Compare importance scores
4. Ask user to confirm if uncertain

Best Practices

API Usage

1. Add User-Agent header:

   -H "User-Agent: ObsidianLocationNotes/0.1.0"
   

   Required by Nominatim usage policy

2. Respect rate limit: 1 request/second maximum

3. Cache results: Don't re-query same location

4. Use countrycodes: Filter results when country known

   &countrycodes=gb,ie  # UK and Ireland only
   

Search Quality

1. Remove special characters: Convert "Old Man of Storr" → "Old Man Storr" if needed

2. Try variations: "Botanic Garden" vs "Botanical Gardens"

3. Include city for businesses: "Dishoom Edinburgh" not just "Dishoom"

4. Check display_name: Verify result matches expected region

Coordinate Format

Convert API strings to numbers:

lat = float(result['lat'])  # "57.5062935" → 57.5062935
lon = float(result['lon'])  # "-6.180885" → -6.180885

Store as array for Obsidian:

location:
  - 57.5062935   # Latitude
  - -6.180885    # Longitude

Common Use Cases

Case 1: Find Coordinates for Landmark

# Query
curl -H "User-Agent: ObsidianLocationNotes/0.1.0" \
  "https://nominatim.openstreetmap.org/search?q=Eiffel+Tower&format=json&limit=1"

# Response
[{
  "lat": "48.8583701",
  "lon": "2.2944813",
  "display_name": "Eiffel Tower, Avenue Anatole France, Paris, France"
}]

# Result
[48.8583701, 2.2944813]

Case 2: Geocode Restaurant with City

# Query with country filter
curl -H "User-Agent: ObsidianLocationNotes/0.1.0" \
  "https://nominatim.openstreetmap.org/search?q=Dishoom+Edinburgh&countrycodes=gb&format=json&limit=3"

# Select best result based on type=restaurant
# Result
[55.9533, -3.1883]

Case 3: Handle Failed Lookup

# Query returns empty array
[]

# Fallback: Leave location empty
location: null  # or omit field entirely

Utility Script

A bash script for coordinate lookup is provided:

scripts/geocode.sh

#!/bin/bash
# Geocode a location using Nominatim API
# Usage: ./geocode.sh "Location Name" [country_code]

LOCATION="$1"
COUNTRY="${2:-}"

# Build query
QUERY="https://nominatim.openstreetmap.org/search"
QUERY+="?q=$(echo "$LOCATION" | sed 's/ /+/g')"
QUERY+="&format=json&limit=5"

if [ -n "$COUNTRY" ]; then
    QUERY+="&countrycodes=$COUNTRY"
fi

# Query API (with rate limit)
sleep 1  # Respect rate limit
RESPONSE=$(curl -s -H "User-Agent: ObsidianLocationNotes/0.1.0" "$QUERY")

# Parse first result
echo "$RESPONSE" | jq -r '.[0] | "\(.lat)\n\(.lon)\n\(.display_name)"'

Usage:

./geocode.sh "Old Man of Storr"
./geocode.sh "Dishoom" "gb"

Validation

Check coordinates are valid:

• ✅ Latitude: -90 to 90
• ✅ Longitude: -180 to 180
• ✅ Minimum 6 decimal places for accuracy
• ✅ Matches expected region (check display_name)

Example validation:

def validate_coordinates(lat, lon):
    if not (-90 <= lat <= 90):
        return False
    if not (-180 <= lon <= 180):
        return False
    return True

Troubleshooting

Issue: Empty results Cause: Location not in OSM or misspelled Fix: Try alternative names, add geographic context, or leave empty

Issue: Wrong location returned Cause: Ambiguous name (e.g., "Springfield" exists in many countries) Fix: Add country/region to query, check display_name

Issue: Rate limit error Cause: Requests too frequent Fix: Add sleep(1) between requests, respect 1 req/second limit

Issue: Coordinates seem inaccurate Cause: OSM data quality or location ambiguity Fix: Verify display_name matches expected location, try structured search

Additional Resources

For detailed API documentation and advanced features:

• references/nominatim-api.md - Complete API reference
• examples/geocode-examples.sh - Working search examples
• scripts/geocode.sh - Utility script for coordinate lookup

Use these resources when working with complex queries or troubleshooting coordinate issues.