Cadastral Map vs Satellite Photo Overlay: Real Encroachment or Error in Nice?

LAND — BOUNDARY SURVEY

You’ve just overlaid your cadastral plan onto a satellite view, perhaps on Géoportail. The boundaries don’t line up. An apparent gap of 30, 50 centimetres, sometimes more. Your first thought: has a neighbour encroached? In Nice, where land pressure is high, this question comes up constantly. What nobody tells you: this gap is often not an encroachment at all. It’s an illusion created by the difference between the cadastral coordinate system and the one used by satellite imagery.

What You See — And What It Really Hides

The symptom is simple: online, you align the cadastral outline (often a PDF, or pulled from the land registry site) with the aerial image. The corners of your plot don’t fall exactly on the visible fences or walls. The gap looks glaring. You measure on screen with the ruler tool: a 40 cm difference. Doubt sets in.

What it actually hides is a georeferencing problem. The historical cadastral plan, especially in Nice’s older neighbourhoods, was often drawn up in the old NTF system. Modern satellite photos, meanwhile, use RGF93, France’s official system today. Switching from one to the other without precise correction introduces systematic errors. On a case in Nice last year, I measured an apparent 55 cm gap on a party wall. After a survey with a total station, the real encroachment was… 7 cm. Most of the offset came from the projection mismatch. In practice, what does that mean? Your on-screen measurement has no legal value whatsoever. It ignores image distortion, pixel resolution, and the true precision of the cadastral plan, which can run close to a full metre in some parts of Nice.

Diagram: apparent offset vs actual ground truth Cadastral boundary (paper/PDF plan) Wall/fence visible (satellite photo) Apparent gap Δ (measured on screen) True position after projective correction (total station survey) ε₁: projection error (NTF → RGF93 system) ε₂: real ground-level gap (potential encroachment)

The Equation That Separates Illusion From Real Problem

To know whether you’re dealing with a projection error or an actual encroachment, you need to break the total measured gap (Δ) into two components: the referencing error (ε₁) and the real gap on the ground (ε₂). The key formula is: Δ = ε₁ + ε₂. Where ε₁ is the error caused by the change of geodetic system and the plan’s own precision. Take a concrete case in Nice, in a neighbourhood like l’Ariane. The cadastral plan dates from the 1970s, in the NTF system. The conversion to RGF93 can induce an ε₁ error in the range of 0.40 m to 0.80 m depending on the area, due to grid corrections. Say ε₁ = 0.55 m. You measure a Δ = 0.62 m on your screen. The potential real gap is ε₂ = Δ − ε₁ = 0.62 − 0.55 = 0.07 m, or 7 cm.

This result must be weighed against the legal tolerance. Article 646 of the French Civil Code, supplemented by Decree No. 2007-1735 (Article 26), sets the framework. It doesn’t give a single universal figure, but case law and surveying practice often use the threshold of a « certain boundary marker. » In practice, for a boundary survey in Nice, a gap under 5 cm is often considered within the margin of uncertainty of older plans, especially where no adversarial boundary survey has ever been carried out. A 7 cm gap starts to become significant, but requires indisputable physical evidence. What this means in concrete terms: your apparent 62 cm gap shrinks, after calculation, to 7 cm. Is it an encroachment? Possibly. But it now needs verifying on the ground with an instrument that eliminates the ε₁ error.

The Consequences of Starting From the Wrong Diagnosis

Acting on a purely visual gap can drag you into a costly, needless neighbour dispute. Imagine sending a formal notice over a gap that doesn’t actually exist. The relationship sours immediately. In a similar case in Nice, an owner started proceedings over a supposed 80 cm gap. The court-ordered expert survey revealed a 73 cm projection error. The real encroachment was negligible. Expert and legal fees: over €4,000, for nothing.

Over the longer term, leaving a genuine encroachment unverified can also work against you. Thirty-year adverse possession can come into play. If a neighbour is actually occupying a strip of your land in Nice, and you don’t challenge that occupation for 30 years, they could acquire ownership of it — even for a few centimetres. There’s a secondary risk too: an incorrect encroachment declaration to the land registry can needlessly complicate your parcel record and cause problems down the line when you come to sell.

Remedies and Guarantees: What You Can Demand

Frankly, forget the on-screen measurement. The only approach that holds up is technical and adversarial. First, have a precise topographic survey carried out by a land surveying expert. In Nice, they’ll use an electronic total station (Leica TS type) or an RTK GNSS receiver, accurate to ±1–2 cm within the RGF93/Lambert 93 system. This survey will pin down the exact position of your boundaries and neighbouring structures. If a significant gap (say, over 5 cm) is confirmed, you open discussions with your neighbour, ideally by registered letter with acknowledgement of receipt. If they refuse, Article 646 of the Civil Code lets you bring the matter before the Nice judicial court to have a boundary survey ordered. Decree 2007-1735 (Article 26) sets out how the court-appointed expert is designated. The expert, typically an accredited land surveying expert, will carry out an adversarial measurement and draw up a boundary report that carries the force of law. It’s the only way to get certainty and an enforceable decision. Once signed by both parties and the expert, the boundary survey becomes binding on third parties and is recorded with the land registry authorities.

Frequently Asked Questions

Why doesn’t my cadastral plan match the satellite photo on Géoportail?

It’s often due to a geodetic system mismatch: the historical cadastral record uses the old NTF system, while modern satellite photos use RGF93. This difference can create an apparent gap of 30 to 80 cm, without there necessarily being any real encroachment.

How do I know if the gap between the cadastral plan and the satellite photo is an encroachment or a projection error?

You need to break down the total gap: the projection error (ε₁) caused by the system change, and the real ground-level gap (ε₂). A land surveying expert can measure ε₁ with a total-station survey. If ε₂ is under 10 cm, it’s probably an illusion. In Nice, the apparent gap is very often not a real encroachment.

How much does a topographic survey cost to check for encroachment in Nice?

For a standard plot in Nice, expect to pay between €800 and €1,500 for a total-station survey with a report. It generally takes 1 to 2 weeks. That cost is modest compared to the expense of a dispute or a buying mistake.

What should I do if I suspect encroachment after spotting a gap on Géoportail?

Don’t rely on the online overlay alone. Call in a land surveying expert for a precise on-site survey. Only they can determine whether the encroachment is real. In the meantime, avoid accusing your neighbour: the gap is very often just a projection illusion.

Does a PDF cadastral plan have legal value to prove encroachment?

No, a PDF cadastral plan has no legal value for establishing property boundaries. It’s indicative only and can be off by several metres. Only a boundary survey carried out by a land surveying expert holds up in court.

Need an expert? See our dedicated page: Land Surveying Expertise and Boundary Disputes — response within 48h across the Alpes-Maritimes and the Var.

📅 Updated 31/08/2026 — By Cassini Expertise, independent building expert covering départements 06 & 83. Free quote: 04 22 46 06 04.

📅 Mis à jour le 25/09/2026 — Cassini Expertise, expert bâtiment indépendant (06·83·13·40).