The Géoportail displays dozens of overlapping map layers, each with its own color palette and symbols. For anyone consulting an IGN map online, a local urban planning plan, or a natural risk layer, the difficulty lies not in accessing the data but in understanding what each color, line, and pictogram actually means.
This article compares the major families of symbols on the Géoportail and highlights the differences in interpretation between topographical layers and regulatory layers.
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Topographical Layers and Regulatory Layers on the Géoportail: Two Logic Systems of Legend
The most common confusion arises from the fact that the Géoportail mixes layers that do not speak the same visual language on the same screen. A classic IGN map uses stable conventions for decades to represent relief, water, or vegetation. An urban planning layer (PLU, RNU) follows color codes defined by legislative and regulatory texts, unrelated to the topographical palette.
| Layer Family | Examples of Symbols | Legend Logic | Legal Value of Display |
|---|---|---|---|
| Topographical (IGN map) | Contour lines, blue line (water), green areas (forest) | Historical cartographic convention | None (description of terrain) |
| Regulatory (PLU, RNU) | Zones U, AU, A, N in distinct colored areas | Codification linked to an urban planning document | Non-binding extracts online |
| Thematic (risks, environment) | Colored zoning, hatching, point icons | Variable according to the data producer | According to the source document |
This table summarizes the central point: a single color area does not mean the same thing from one layer to another. A green on the IGN map indicates vegetation. A green on an urban planning layer may correspond to a natural area that cannot be built on. Reading the legend of each activated layer is the only way to avoid misunderstanding.
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To deepen the understanding of urban planning layers specifically, a detailed guide on the legend on the Géoportail breaks down the color codes zone by zone.
IGN Symbols on the Géoportail: Relief, Water, and Administrative Boundaries
The IGN topographic map remains the most consulted base map on the Géoportail. Its symbols follow conventions shared by most hiking and terrain maps.
Contour Lines and Relief
Contour lines appear in orange or light brown. Their spacing indicates the slope: closely spaced lines indicate steep terrain, while spaced lines indicate gentle relief. The main contour lines, which are thicker, carry a numbered altitude. The marked points provide additional information by indicating the exact altitude of peaks or passes.
Hydrographic Network and Wetlands
Everything that is blue on the IGN map represents water. A thin blue line represents a stream, while a wider line represents a river. Light blue areas correspond to bodies of water (lakes, ponds, reservoirs). Wetlands are often represented by horizontal blue hatching on a white background.
Boundaries and Notable Points
Administrative boundaries (communes, departments, regions) appear as dashed lines in different colors. Points of interest (churches, town halls, shelters) are represented by standardized pictograms. Marked hiking trails are shown as colored dashes according to the type of marking GR, GRP, or PR.
- Thick solid black line: main road or building
- Dotted purple or pink line: higher-level administrative boundary (department, region)
- Cross pictogram: religious building, often used as a landmark in hiking
- Triangle with altitude: peak with altitude, a fundamental reference for terrain orientation
Colors of Urban Planning Zones: What the Géoportail Displays Without Legal Guarantee
The Géoportail of urban planning overlays the IGN base maps with zoning from municipal urban planning documents. The colors follow a coding specific to each type of zone, but the online display does not constitute a binding document.
U zones (urban) generally appear in red or orange hues. AU zones (to be urbanized) adopt yellows. A zones (agricultural) are often in brown or ochre. N zones (natural and forested) are in green. This palette varies slightly from one PLU to another, as each commune can adapt its conventions within the regulatory framework.
The trap lies in the overlay. When the PLU layer is activated over the IGN map, the colors visually mix. A yellow area of AU zone placed over a green forest background produces an ambiguous hue. Disabling the topographical background allows for reading the zoning without color interference.
Thematic Layers and New Symbols: ZFE, Natural Risks, Biodiversity
The Géoportail is no longer limited to terrain maps and urban planning documents. Recent thematic layers add symbols that did not exist in the classic cartographic vocabulary.
On Google Maps, a “Z” symbol now indicates low emission zones (ZFE) and allows access to details of local traffic constraints. The Géoportail, on the other hand, offers environmental layers (Natura 2000 zones, protection perimeters, risk zoning) whose legends vary according to the data producer.
Each thematic layer has its own legend, accessible by clicking on the information icon associated with the layer in the sidebar. This legend is not displayed by default, which explains the perplexity of many users faced with color areas without visible explanation.
- Red or orange hatching on a risk layer: high hazard zone (flood, landslide)
- Green areas with borders on an environmental layer: ecological protection perimeter
- Point icons (triangles, circles): classified sites, historical monuments, or measurement points
The Géoportail is also evolving towards cartes.gouv.fr, presented as the new public service for maps and territorial data. This platform change does not alter the legend logic but redistributes the interface for accessing layers and their metadata.
The distinction between what a symbol shows and what it is legally worth remains the main point of vigilance. A zoning displayed online may be outdated or simplified compared to the official document available at the town hall. For a real estate purchase project or a building permit application, verification with the municipal urban planning service remains the only reliable step.