The growing use of orthophoto maps from the French National Institute of Geographic and Forest Information (IGN) in construction and renovation projects in Nice regularly raises the question of their consistency with the official cadastral plan. When a planimetric discrepancy appears between these two geographic reference systems, determining property boundaries and complying with planning regulations can become problematic. Cassini Expertise, an independent building expert working across Antibes, Nice and the Alpes-Maritimes, examines the technical causes of these divergences and their legal implications.
Nature and origin of planimetric discrepancies between mapping sources
The cadastral plan and IGN orthophoto maps are two distinct cartographic products, produced using different methodologies and serving different purposes. The cadastre, managed by the French Directorate General of Public Finances (DGFiP), is primarily intended for the tax identification of land parcels and the representation of their ground footprint. IGN orthophoto maps, by contrast, result from photogrammetric processing of aerial photographs, designed to provide a georeferenced image of the territory.
These differences in design explain the planimetric discrepancies frequently observed. The cadastral plan, some sections of which date back several decades without a full update, can present geometric distortions inherited from older surveying techniques. Orthophoto maps, even when geometrically rectified, contain residual inaccuracies linked to the digital terrain model used, optical distortions, and the conditions under which the images were taken.
Projection systems and geodetic datum
Overlaying these two reference systems requires expressing them in a common coordinate system. The digitised cadastral plan is being progressively migrated to the RGF93 reference system (Réseau géodésique français 1993), Lambert-93 projection. IGN orthophoto maps are also distributed in this system. However, the cadastre’s digitisation history — sometimes carried out by converting from older local systems — can generate local distortions.
In Nice, as across the Alpes-Maritimes department, the former Lambert South zone system was long in use. The conversion process to Lambert-93, although rigorous, does not always eliminate the distortions inherent in the old cadastral survey sheets. These distortions can reach several tens of centimetres, or even more in certain areas where the geodetic control network was insufficiently densified.
Legal implications of discrepancies between mapping sources
From a legal standpoint, the cadastral plan does not constitute a title of ownership and does not officially define property boundaries that are enforceable between neighbours. Article 1 of Decree No. 55-471 of 30 April 1955 states that the cadastral plan is drawn up solely for the purposes of the assessment and apportionment of property tax. Established case law consistently confirms that only a boundary survey (bornage), a legal operation carried out by a chartered surveyor (géomètre-expert), establishes property boundaries with evidential value.
When a dispute arises concerning the positioning of a building relative to property boundaries, the judge cannot rely exclusively on the cadastral plan or the IGN orthophoto. The Grasse regional court (with jurisdiction over Antibes and Nice), the Nice judicial court, or the Toulon judicial court (depending on the property’s location) will generally order a judicial expert assessment to establish the actual extent of the land and any easements.
Planning permits and cartographic reference systems
The review of building permit applications or prior planning declarations is based on the site and layout plans submitted by the applicant. Nice’s planning department uses the cadastral reference system to verify compliance with the local urban plan (PLU), particularly setback requirements, positioning relative to boundary lines, and ground coverage.
A planimetric discrepancy between the orthophoto and the cadastre can lead to an incorrect assessment of a project’s compliance. If the administrative review is based on the cadastre while the actual situation on the ground — verifiable via the orthophoto or a topographic survey — reveals a different picture, the applicant may challenge a refusal decision or any conditions imposed. Conversely, a building constructed in accordance with the site plan may later prove non-compliant if the cadastral plan contained a representational error.
The building expert’s methodology for analysing discrepancies
When faced with a divergence between an IGN orthophoto and the cadastral plan, an independent building expert carries out a methodical analysis of the sources of error and the reference systems used. This technical approach aims to identify the origin of the discrepancy and assess its consequences for the construction project or the dispute at hand.
Reviewing metadata and expected accuracy
The expert first checks the technical specifications of the documents used. IGN orthophoto maps are published with accuracy specifications that vary according to resolution (from 5 cm to 50 cm per pixel). Planimetric accuracy, generally expressed as a standard deviation, depends on the digital terrain model used for orthorectification. In dense urban areas such as Nice, buildings of varying heights create residual parallax effects that can reach several tens of centimetres.
The digital cadastral plan, for its part, has variable accuracy depending on the age and original survey methods. Sections recently updated using GPS surveys offer centimetre-level accuracy, while sections digitised from paper survey sheets may contain inaccuracies ranging from decimetres to metres. The expert consults DGFiP archives to identify the date and survey method used for the relevant sections.
Joint survey and geodetic referencing
When the stakes justify it, the expert recommends a joint topographic survey carried out by a chartered surveyor. This professional establishes a link to the national geodetic network and determines the coordinates of key reference points (building corners, boundary fences, road features) within the RGF93 Lambert-93 system. This operation makes it possible to objectively compare the three reference sources: the site itself, the cadastre, and the orthophoto.
The topographic survey, distinct from a formal boundary survey (bornage), provides a reliable geometric representation of the existing situation. It helps identify whether the observed discrepancy stems from a distortion in the cadastre, an inaccuracy in the orthophoto, or a divergence between the actual situation and the cadastral record (an undeclared structure, or a boundary change that was never recorded).
Case studies of planimetric discrepancies in Nice
The situations encountered by Cassini Expertise across the Alpes-Maritimes illustrate the range of problematic configurations. In several older districts of Nice, the cadastre is based on 19th-century surveys that were digitised without a full update. Recent orthophotos reveal systematic discrepancies that can exceed one metre, particularly in steep areas where projection distortions amplify inaccuracies.
In more recently built residential areas, the discrepancy more often results from a divergence between the authorised situation (as recorded in the cadastre) and what was actually built. An undeclared extension, a fence positioned differently from the original project, or a building placement error can all create a mismatch between the orthophoto (which reflects the site as it actually is) and the cadastre (which reflects the administrative declaration).
Boundary disputes between neighbours and determining property lines
When a property owner invokes the IGN orthophoto to challenge the positioning of a neighbouring building, the expert points out that this document carries no legal weight. Only a boundary survey (bornage), carried out jointly by a chartered surveyor and recorded in an official report, establishes property boundaries that are enforceable against third parties. The boundary survey relies on title deeds, possession, plans attached to deeds and, as a secondary source, the cadastre as a simple presumption.
The building expert steps in upstream to characterise the factual situation: the actual footprint of the building, compliance with regulatory setback distances from apparent boundaries, and conformity with the planning permission granted. This technical analysis clarifies for the parties the nature of the dispute (encroachment, administrative non-compliance, or a simple discrepancy in cartographic representation) and points towards the appropriate procedure: an amicable or judicial boundary survey, an action for demolition, or administrative regularisation.
The expert’s recommendations and corrective steps
When faced with a significant planimetric discrepancy, the independent building expert makes recommendations tailored to the situation. If the discrepancy stems from a distortion in the cadastral plan, a request for an update can be submitted to the DGFiP cadastral service. This procedure, which is free of charge, requires a survey document (document d’arpentage) prepared by a chartered surveyor in accordance with the order of 16 September 2003.
When the divergence results from a building that does not comply with its planning permission, or that was never declared, administrative regularisation becomes necessary. The property owner must file a modified building permit or a regularisation permit application, accompanied by an accurate site plan prepared by a qualified professional. The review of this application checks the compliance of the actual project against current planning regulations.
Judicial expert assessment and establishing proof
In proceedings before the Grasse, Nice or Toulon judicial court, the judge may order an expert assessment to establish the actual extent of the land and characterise the discrepancy found. The judicial expert carries out a joint on-site inspection, analyses the planning and cadastral documents, and may call on a chartered surveyor to carry out a precise topographic survey.
The judicial expert report is a key piece of evidence for the judge. It allows a determination of whether a building genuinely encroaches on the neighbouring parcel, whether regulatory setbacks are respected, and whether the planning permission was correctly carried out. On appeal, the Aix-en-Provence Court of Appeal re-examines these technical elements to confirm or overturn the first-instance decision.
Professional liability and applicable guarantees
When a positioning error results from a construction professional’s misreading of the cartographic reference sources, the question of their liability may arise. The architect who designs the project and the contractor who builds it are bound by an obligation of result regarding compliance with the dimensions and positioning set out in the execution drawings.
If the positioning error results in non-compliance with the planning permission or an encroachment on the neighbouring parcel, the client may invoke the contractor’s contractual liability under Article 1231-1 of the French Civil Code. Defects affecting the structural soundness of the building or rendering it unfit for its intended purpose fall under the ten-year structural warranty (garantie décennale). Less severe defects may be covered by the one-year completion warranty (garantie de parfait achèvement) or the two-year functional warranty (garantie biennale de bon fonctionnement), depending on their nature.
The independent building expert’s role in establishing liability
An independent building expert such as Cassini Expertise, working in Antibes and across the Alpes-Maritimes (06) and Var (83), analyses the technical causes of the positioning error and identifies any professional failings involved. This analysis distinguishes between a design error (an incorrect site plan, poor use of cadastral documents), an execution error (on-site positioning that does not match the drawings), and a cartographic representation error (an inherent inaccuracy in the cadastre or the orthophoto).
The technical expert report allows the client to quantify the loss suffered: the cost of demolition and reconstruction, the reduction in the property’s value, or the inability to obtain compliant planning permission. These elements form the basis of a contractual liability claim brought before the competent judicial court. The contractor’s ten-year structural insurance is called upon if the defects fall under that warranty; otherwise, professional civil liability insurance applies.
For any question relating to planimetric discrepancies between IGN orthophotos and the cadastral plan in Nice or across the Alpes-Maritimes and Var, Cassini Expertise, an independent building expert based in Antibes, can be reached on 04 22 46 06 04. The firm does not carry out boundary surveys, which are reserved for chartered surveyors, but directs clients to the right professionals for each type of technical or legal matter.