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TOPOGRAPHY — GEOMATICS
You consult Géoportail to check the boundaries of your property in Nice. You overlay the cadastral map (land registry) on the IGN orthophoto. And then, surprise: your fence seems offset by 30 centimeters from the digital boundary. You zoom in, you check the address. The discrepancy is indeed there. This offset, which many take for a simple display error, actually hides a georeferencing problem that can affect your property rights. In Nice, where the price per square meter often exceeds €6,000, 30 cm represents a non-negligible area.
What You See — and What It Really Hides
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The symptom is simple: on your screen, the cadastral lines and the visible edges on the aerial photo do not perfectly overlap. You think it’s a site bug. In practice, what does it look like? Frankly, what nobody says: the IGN orthophoto and the digital cadastral map are two distinct products, generated at different times and with different methods. The cadastral map, derived from sometimes updated old surveys, and the orthophoto, a georeferenced rectified image, do not necessarily have the same absolute anchor point in space.
What I often see in this case, especially on files in Nice last year, is that owners measure the on-screen offset with a virtual ruler. They get a figure, for example 0.30 m. But this screen measurement is not a field measurement. It does not account for the actual scale, potential image distortions, or the coordinate system used. The real question is: does this apparent offset correspond to a real physical offset on the ground, measurable with a precision instrument? That’s where the complexity begins.
The Calculation That Reveals the Real Error
To quantify the error, we don’t rely on the screen. We calculate the Root Mean Square Error (RMSE) between ground control points and their position on both documents. The formula is: RMSE = √(Σ(ΔX² + ΔY²) / n), where ΔX and ΔY are the coordinate differences at each point, and n is the number of points. Let’s take a concrete case measured in Nice with a GNSS RTK (precision ±1 cm). On 5 parcel corner points, we record the differences between the real position (measured in RGF93/Lambert 93) and the position on the digital cadastral map. Assume the following differences in meters: Point1 (ΔX=0.25, ΔY=0.18), Point2 (0.30, 0.22), Point3 (0.28, 0.15), Point4 (0.32, 0.20), Point5 (0.27, 0.19).
Let’s calculate:
Σ(ΔX² + ΔY²) = (0.25²+0.18²) + (0.30²+0.22²) + (0.28²+0.15²) + (0.32²+0.20²) + (0.27²+0.19²)
= (0.0625+0.0324) + (0.09+0.0484) + (0.0784+0.0225) + (0.1024+0.04) + (0.0729+0.0361)
= 0.0949 + 0.1384 + 0.1009 + 0.1424 + 0.109 = 0.5856
RMSE = √(0.5856 / 5) = √(0.11712) ≈ 0.342 m.
This result of 34 cm is the average positioning error of the cadastral map relative to ground truth. The CC3D standard (technical specifications for geospatial data) sets tolerances depending on scale. For data at 1:1000 scale, the planimetric tolerance is often around 0.50 m. Here, 0.34 m is below that threshold, but it far exceeds the precision expected for boundary marking or a construction project in Nice where margins are tight. Concretely, this means the official document has a systematic inaccuracy. If you rely solely on it to install a fence, you risk encroaching on your neighbor’s land.
The Consequences If You Ignore This Offset
In the short term, nothing moves. Your house is still there. But this latent offset becomes a problem the day you undertake construction work. Imagine: you want to extend your villa in the Cimiez district of Nice. The architect uses the cadastral map to comply with legal distances. If the map is offset by 34 cm, your extension could actually encroach on a right-of-way or fail to respect the minimum distance of 3 meters from the boundary. The neighbor contests. Work stops.
In the longer term, say 2 years, the neighborly conflict escalates. Legal fees add up. A court-ordered expert appraisal is mandated by the Nice court. The expert, with their Leica TS total station, confirms the encroachment. The secondary pathology? Not only must you demolish the overbuilt part (cost: €15,000 to €40,000), but you also face damages and legal costs. Moreover, the cadastral map’s inaccuracy may be systematic, meaning other boundaries could also be wrong. A full survey of the property becomes necessary, costing several thousand euros. In short, ignoring a 30 cm offset can lead to a financial and legal nightmare.
📅 Mis à jour le 25/08/2026 — Par Cassini Expertise, expert bâtiment indépendant 06 & 83. Devis gratuit : 04 22 46 06 04.
Questions fréquentes
What is the difference between IGN orthophoto and cadastral map?
IGN orthophotos are aerial images corrected for terrain distortion, showing the Earth's surface as it appears, while cadastral maps are legal documents showing property boundaries and land parcels. The main difference is that orthophotos are visual and current, whereas cadastral maps are authoritative for ownership but may have positional inaccuracies.
How accurate is IGN orthophoto for property boundaries?
IGN orthophotos typically have a planimetric accuracy of about 30 cm, meaning features can be offset by up to 30 cm from their true ground position. This is not precise enough for legal boundary determination, as it can lead to disputes; for that, a licensed surveyor is required.
Why is there a discrepancy between orthophoto and cadastral map in Nice?
Discrepancies arise because orthophotos are based on recent aerial imagery with a 30 cm error, while cadastral maps are often older and digitized from paper maps with varying accuracy. Additionally, terrain, building overhangs, and map projection issues can cause offsets, making it essential to consult a surveyor for precise measurements.
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