GIS QGIS and Spatial Analysis for Essonne Local Authorities

An Essonne municipality wants to know how many dwellings lie within 300 metres of a flood-prone zone. An inter-municipal authority is looking for the best parcels for a future development zone. A 91 parks-and-gardens department wants to quantify the effect of a planting plan on urban heat islands. Three questions, one underlying tool: GIS spatial analysis. This article details how I tackle this type of problem with QGIS, PostGIS and geospatial Python, for local authorities and engineering consultancies across Île-de-France.

I run CODRUM (SIREN 103 982 005, RCS Évry), a geomatics and web development consultancy based in Morsang-sur-Orge (Essonne, 91). The firm builds on the academic research I conduct as part of the Master 1 Geomatics G2M programme (multi-criteria spatial analysis, Sentinel-2 remote sensing, LiDAR HD IGN, spatial epidemiology) and supports both local Essonne projects and international studies. The goal here is not to stack technical acronyms, but to show concretely what a GIS analyst does and when it becomes worthwhile to call on a specialist consultancy.

Desktop GIS or cloud GIS: which tool for which need?

Before moving on to use cases, a clarification is needed. Two families of GIS tools coexist today, and the choice between them shapes both the final invoice and the long-term viability of the deliverable.

Desktop GIS (QGIS primarily, ArcGIS Pro on the proprietary side) remains the reference for one-off analyses, polished cartographic output and heavy local processing. This is what I use for 80 per cent of assignments: QGIS 3.x opens everything, processes everything, exports everything, and costs nothing in licensing. Ideal for a local urban plan, a territorial diagnosis or a one-off regulatory file.

Cloud GIS (hosted PostGIS, ArcGIS Online, GeoServer, Leaflet or Mapbox web maps) takes over when the data must live: frequent updates, multi-user consultation, integration into a website. For a town hall that wants an internal cartographic portal consultable by its services, this is the right architecture. In practice, the majority of serious projects combine the two: PostGIS to store and structure, QGIS to analyse and produce maps, and a lightweight web layer for dissemination.

Concrete use cases for Essonne local authorities

Essonne concentrates a rare diversity of planning challenges: highly urbanised areas in the north, preserved agricultural land in the south, sensitive valleys (Orge, Yvette, Juine), and major development projects (Plateau de Saclay, Grand Paris). Here are the assignments I encounter most often.

Local urban plan revision and territorial diagnosis. Overlay of cadastral layers, regulatory zoning, protection perimeters (historic monuments, Natura 2000, water-supply catchments) to produce baseline maps. Highly standardised work, well suited to automation through reproducible QGIS models.

Development zone preparation and planning projects. Multi-criteria analysis to locate the best footprints (transport proximity, available land, environmental constraints, minimum surface area). A topic where PostGIS shines thanks to its complex queries on large extents.

Green plans and network management. Canopy characterisation from LiDAR HD IGN, mapping of urban heat islands, prioritisation of areas to revegetate. On the asset-management side, structuring road networks, public lighting, urban furniture and street trees in PostGIS: tablet-based field capture, online consultation by municipal services.

Case study: spatial epidemiological analysis in Colombia

To illustrate concretely the technical depth of a well-conducted GIS project, I refer to the spatial analysis of dengue fever in Girardot, a Colombian city where I modelled the environmental determinants of viral transmission.

The project combined QGIS for final cartographic production and PostGIS for the analytical layer. Key overlays used ST_Buffer to generate buffer zones around suspected larval breeding sites, then ST_Intersects to identify exposed dwellings within a defined radius. A Python stack (geopandas, shapely) automated aggregation by statistical block and the production of risk indicators.

The final output relied on QGIS cartographic layouts: thematic maps with district-level choropleths, overlays of environmental variables (vegetation cover, built-up density, access to running water), multi-page output ready for printing. The methodology is transferable to Île-de-France issues: air pollution, noise exposure, accessibility to health services, vulnerability to heat.

Technical stack, methodology and pricing

My standard working environment rests on three complementary building blocks. QGIS 3.x for cartography, the graphical modeller and print layouts. PostGIS for structured storage and spatial queries at scale. Geospatial Python (geopandas, shapely, GDAL, rasterio) for scripted, reproducible processing chains.

The methodology comes in five steps: audit of existing assets and data, modelling of the spatial schema, analysis proper, cartography of the results, then delivery with full documentation. Final deliverables are issued in open, long-lived formats: Geopackage (recommended), Shapefile (backward compatibility), GeoTIFF for rasters, and where appropriate a Leaflet web map for online dissemination.

On indicative pricing, a targeted cartographic audit starts from 800 €, while a full GIS study(scoping, data acquisition, analysis, cartography, deliverables) starts from 2 500 €. The final price depends on the geographic scope, the number of data sets and the level of deliverables expected: personalised quote within 48 hours after a free initial scoping call.

For a full breakdown of every service and price range, see the Services & Pricing page.

When should you bring in an independent GIS analyst?

In practice, as soon as the question contains a geographic where or how many, a GIS is relevant. A municipality preparing its sustainable development report, an environmental consultancy assembling a water-law file, an association documenting land-use pressure on a natural area: all benefit from outsourcing this dimension to a specialist rather than improvising in Excel or on Google Maps.

If you have a project, even loosely defined, the simplest next step is to get in touch for an initial 30-minute video scoping call with no commitment. That is often enough to identify whether the need calls for a light audit, a full study or simply a one-off helping hand.