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Feature Querying with WFS

Scenario: You need to retrieve raw geographic features from a Qarta layer for analysis, export, or downstream processing. You want to filter by location (bounding box) or attributes (CQL filter), and retrieve the data as GeoJSON.

Workflow:

  1. Use DescribeLayer to find the WFS typeNames for a layer
  2. Call GetFeature with filters and output format
  3. Parse GeoJSON and process features in your application
  4. Paginate through large result sets if needed

Step 1: Get Layer Metadata​

Use the WMS DescribeLayer operation to find the corresponding WFS layer name:

curl -X GET "https://graph.quarticle.ro/graph/layers/wms/DescribeLayer" \
  -H "Authorization: YOUR_API_KEY" \
  -G \
  -d "service=WMS" \
  -d "version=1.1.1" \
  -d "request=DescribeLayer" \
  -d "layers=GRAPHRASTER:cities"

From the response, extract the WFS typeNames (usually same as the layer name).

Step 2: Query Features by Bounding Box​

Retrieve all features within a geographic area:

# Cities in New York area (bbox in EPSG:4326)
curl -X GET "https://graph.quarticle.ro/graph/layers/wfs/GetFeature" \
  -H "Authorization: YOUR_API_KEY" \
  -G \
  -d "service=WFS" \
  -d "version=2.0.0" \
  -d "request=GetFeature" \
  -d "typeNames=GRAPHRASTER:cities" \
  -d "bbox=-74.1,40.6,-73.9,40.8,EPSG:4326" \
  -d "outputFormat=application/json" \
  | jq '.features[] | .properties'

Step 3: Query Features by Attributes (CQL Filter)​

Filter features using attribute conditions:

# Cities with population > 1 million
cql_filter=population > 1000000

Step 4: Combine Spatial and Attribute Filters​

Filter by both location and attributes:

curl -X GET "https://graph.quarticle.ro/graph/layers/wfs/GetFeature" \
  -H "Authorization: YOUR_API_KEY" \
  -G \
  -d "service=WFS" \
  -d "version=2.0.0" \
  -d "request=GetFeature" \
  -d "typeNames=GRAPHRASTER:cities" \
  -d "bbox=-74.1,40.6,-73.9,40.8,EPSG:4326" \
  -d "cql_filter=population > 100000" \
  -d "propertyName=name,population,geometry" \
  -d "outputFormat=application/json"

Step 5: Paginate Through Large Result Sets​

For large datasets, paginate to avoid timeouts:

async function getAllFeatures(typeNames, cqlFilter = null) {
  const apiKey = process.env.QARTA_API_KEY;
  const pageSize = 100;
  let allFeatures = [];
  let page = 0;

  while (true) {
    const params = new URLSearchParams({
      service: 'WFS',
      version: '2.0.0',
      request: 'GetFeature',
      typeNames: typeNames,
      count: pageSize.toString(),
      startIndex: (page * pageSize).toString(),
      outputFormat: 'application/json'
    });

    if (cqlFilter) {
      params.append('cql_filter', cqlFilter);
    }

    const response = await fetch(
      `https://graph.quarticle.ro/graph/layers/wfs/GetFeature?${params}`,
      { headers: { 'Authorization': `${apiKey}` } }
    );

    const geojson = await response.json();
    const count = geojson.features.length;
    allFeatures.push(...geojson.features);

    console.log(`Page ${page + 1}: Retrieved ${count} features (total: ${allFeatures.length})`);

    // Stop if fewer features than page size
    if (count < pageSize) break;
    page++;
  }

  return allFeatures;
}

const allCities = await getAllFeatures('GRAPHRASTER:cities', "population > 50000");
console.log(`Total: ${allCities.length} cities`);

Step 6: Export to CSV or File​

Save queried features to a file:

async function exportFeaturesToJSON(typeNames, filename) {
  const features = await getAllFeatures(typeNames);

  const geojson = {
    type: 'FeatureCollection',
    features: features
  };

  const json = JSON.stringify(geojson, null, 2);
  const blob = new Blob([json], { type: 'application/json' });
  const url = URL.createObjectURL(blob);

  const link = document.createElement('a');
  link.href = url;
  link.download = filename;
  link.click();
}

exportFeaturesToJSON('GRAPHRASTER:cities', 'cities.geojson');

Step 7: Spatial Queries (Advanced)​

Use CQL spatial functions to query features:

# Cities within 10km of Times Square
cql_filter=DWITHIN(geometry, POINT(-73.9857 40.7580), 10000, meters)

Complete Workflow Example​

const API_KEY = process.env.QARTA_API_KEY;

async function analyzeMetropolitanAreas() {
  console.log('Querying metropolitan areas...');

  // Step 1: Get all cities
  const params = new URLSearchParams({
    service: 'WFS',
    version: '2.0.0',
    request: 'GetFeature',
    typeNames: 'GRAPHRASTER:cities',
    cql_filter: "type = 'metropolitan' AND population > 1000000",
    propertyName: 'name,population,area,country',
    sortBy: 'population D',
    outputFormat: 'application/json'
  });

  const response = await fetch(
    `https://graph.quarticle.ro/graph/layers/wfs/GetFeature?${params}`,
    { headers: { 'Authorization': `${API_KEY}` } }
  );

  const geojson = await response.json();

  // Step 2: Analyze
  const analysis = {
    totalCities: geojson.features.length,
    totalPopulation: 0,
    averagePopulation: 0,
    cities: []
  };

  geojson.features.forEach(feature => {
    const { name, population, area, country } = feature.properties;
    analysis.totalPopulation += population;
    analysis.cities.push({
      name,
      population,
      area,
      country,
      density: (population / area).toFixed(0)
    });
  });

  analysis.averagePopulation = Math.round(analysis.totalPopulation / analysis.totalCities);

  // Step 3: Export
  const json = JSON.stringify(analysis, null, 2);
  const blob = new Blob([json], { type: 'application/json' });
  const url = URL.createObjectURL(blob);
  const link = document.createElement('a');
  link.href = url;
  link.download = 'metropolitan_analysis.json';
  link.click();

  console.log('Analysis complete:', analysis);
  return analysis;
}

analyzeMetropolitanAreas();

Use Cases​

  • Real estate analysis: Query properties in specific areas and price ranges
  • Environmental data: Download pollution, climate, or vegetation data for analysis
  • Urban planning: Extract zoning, infrastructure, or demographic data
  • Research: Bulk-download geographic features for spatial analysis
  • Data pipeline: Integrate WFS queries into ETL processes

Next Steps​