Slope, height and aspect: what the official terrain model tells you about a plot in Spain

A listing photo taken from the terrace shows the view. It does not show how steeply the ground falls away below the pool, which way the slope faces or how high the house stands above the sea. Spain has an official answer to all three.

The National Geographic Institute (Instituto Geográfico Nacional, IGN) keeps a terrain model built from laser flights. Any plot's shape can be read from it. The data are free to use. They also have limits, and those limits show most on exactly the kind of hillside plot that comes with a view.

What is Spain's official terrain model?

A grid of ground heights for the whole country, measured from aircraft and cleaned of trees and buildings.

The programme behind it, PNOA-LiDAR, aims to capture three-dimensional information for the whole of Spain with LiDAR sensors carried on aircraft, flown in successive surveys of the country. A laser scans the ground from the aircraft and returns a cloud of measured points, which are then classified and edited. Only one class goes into the terrain model: class 2, ground (terreno). From these ground points the IGN generates the digital terrain model (Modelo Digital del Terreno, MDT) with a grid spacing of 2 or 5 metres, depending on the area, and the result shows the ground without obstacles such as woodland or buildings. Each square of the grid carries one height. Where a flight returned no ground points, the IGN fills the gap by interpolation, a calculation from the neighbouring values.

Only after the 2 and 5-metre models have been generated and checked does the IGN produce the derived products MDT25 and MDT200, with grid spacings of 25 and 200 metres. For the third survey, the download centre of the CNIG (Centro Nacional de Información Geográfica) offers a terrain model with a 0.5-metre grid.

1. LiDAR flight0.5 to 5 points per m²the aircraft scans the ground and returns a point cloud
2. Ground classclass 2points classified as terrain, without trees or buildings
3. Terrain model2 or 5 m gridone height per cell, gaps filled by interpolation
4. Derived models25 and 200 m gridproduced once the finer models have passed quality control
5. Slope and aspectper cellslope from the IGN slope model in degrees, aspect calculated with GIS software

Heights are orthometric, meaning heights above the sea-level reference. The reference system is ETRS89 on the mainland, the Balearic Islands, Ceuta and Melilla, and REGCAN95 in the Canary Islands. Both are compatible with WGS84. The files come in an open format, COG (Cloud Optimized GeoTIFF). Using IGN products means accepting a CC-BY 4.0 licence. Under Orden FOM/2807/2015 the data may be used free of charge, provided their origin and ownership are stated.

How precise is the height of a single point?

That depends on which survey the points come from.

The first survey ran from 2009 to 2015 with a uniform 0.5 points per square metre. The second ran from 2015 to 2021 with 0.5 to 4 points per square metre depending on the area, and 14 in Navarra. The third is being captured at a uniform 5 points per square metre; outside Catalonia it began in 2023. Accuracy rose with density. The height error (RMSE Z, the root mean square error in height) fell from 40 cm in the first survey to 10 cm in the third. The second survey has values per area. The IGN's block table lists 0.15 m at 1 point per square metre for the Balearic Islands and 0.2 m at 0.5 points per square metre for Valencia.

These values describe the point cloud the models are made from, and the IGN's documents give no separate accuracy figure for the derived MDT25, so the precision of a single 25-metre cell is not stated anywhere officially. That model was interpolated from the ground class of the PNOA flights, and according to the download centre its coverage for the second survey is still to be completed.

Water is a weak spot. Over lakes, reservoirs and the sea, LiDAR returns few points, so these areas are interpolated and given a constant height. The download centre rates their reliability as low. For a plot on a shoreline, beside a reservoir or with a large pond, read the model height with caution.

How are elevation, slope and aspect worked out?

Elevation is the simple one.

Each cell of the terrain model carries a height above sea level, so the value for a plot can be read off directly without any further calculation, either in a viewer or from the downloaded file. Slope describes how steeply the ground falls. The IGN publishes its own slope model (Modelo Digital de Pendientes, MDP), generated from the terrain model. Each cell stores the slope in degrees against the horizontal plane.

Aspect (orientación) is the compass direction a slope faces as it falls away. Here the official offer stops. The CNIG does not publish a ready-made aspect model; according to its own answers to frequent questions, aspect can be derived from the elevation models with suitable GIS software. Any aspect figure you meet in a report or a listing has been calculated by whoever states it.

You can look up heights without downloading anything. The CNIG names two IGN applications for this:

• Iberpix, a map viewer published by the CNIG and the IGN for viewing maps and layers of geographic information.
• SIGNA, the IGN's web GIS, which among other things calculates longitudinal profiles, with no software to install and no registration.

A longitudinal profile shows the height of the ground along a line you draw. Drawn from the access road to the back boundary, it shows whether the plot climbs evenly or drops away behind the house.

For larger work the download centre offers terrain models at 5, 25 and 200 metres, surface models (MDS), slope models and the LiDAR data themselves.

A Claronda property check reads these values for one plot from the 25-metre terrain model and gives the source and date with each. It starts from the cadastral reference, the code that identifies every plot in Spain; how to read it is set out in the Spanish cadastral reference.

How much of a plot does one grid cell cover?

At a 25-metre grid, one cell stands for an area of 25 by 25 metres. A small plot can lie entirely inside a single cell.

On a hillside that matters. A bank along the boundary, a short steep drop below a terrace or a cut for the driveway disappears into one value for the whole cell. The value describes the hillside more than the plot. For detail on a slope the finer models with 5 or 2-metre grids are a better basis, and where the third survey is available, the 0.5-metre model.

Because the terrain model leaves out buildings and trees, its height is that of the ground beneath them.

Where do height and slope appear in official rules?

In two places that matter to a buyer: the energy requirements for buildings and the official work on slope movements.

Height and the climate zone

Spain's technical building code (Código Técnico de la Edificación, CTE) was introduced by Real Decreto 314/2006. Its basic document on energy saving (Documento Básico de Ahorro de Energía, DB-HE), last amended by Real Decreto 450/2022, sets the climate zone of a site by its province and its height above sea level. The assignment is a table in the annex. You read it with the plot's height.

The DB-HE calculation must also take into account the design, siting and orientation of the building. That refers to the building rather than the slope, although on a sloping plot the two are closely linked. For any specific plot, the table in the current DB-HE is authoritative.

Slope and ground movements

The IGN names terrain height as a basis for, among other things, mapping flood zones and predicting landslides. Flood zones have their own official maps, covered in the guide to flood risk on Spanish property.

The Spanish geological survey (Instituto Geológico y Minero de España, IGME) treats slope in its guide on slope movements as one of several natural contributing factors. It names slopes with a medium or high gradient. Alongside slope it lists rock type and geological structure, the physical properties of the materials, groundwater conditions and climate. According to the guide, susceptibility comes from overlaying at least three main factors: slope, rock type and water. Slope alone does not settle it. A steep plot on solid rock may be assessed quite differently from a gentler one on soaked loose ground.

As an example, the guide proposes three slope classes: below 10°, from 10° to 30°, and above 30°. The IGME also keeps BDMOVES, a national inventory of ground movements such as landslides, rockfalls and subsidence, and in 1987 it published an overview map of ground movements in Spain at a scale of 1:1,000,000. Both record events and areas. For a single plot they do not replace an assessment by a geologist or geotechnical engineer.

How do the properties in Claronda's reports compare?

Most sit low and almost flat. Terrain values from the 25-metre MDT are available for 466 properties with a Claronda report (as of 30.09.2026).

By height, 196 lie below 50 m above sea level, 183 between 50 and under 200 m, 45 between 200 and under 500 m, and only 42 at 500 m or higher, which leaves higher ground in a clear minority. Half of the properties lie below 64 m. The highest stands at 978 m. The slope figures are even more one-sided. 333 properties slope less than 5 %, 111 between 5 and under 15 %, and 20 between 15 and under 30 %. Only 2 reach 30 % or more. These class limits are Claronda's own grouping, not official steps.

Aspect needs a slope to exist.

On almost level ground there is no meaningful direction in which it falls. The point below which ground counts as too flat for a direction is a setting Claronda chose; there is no official threshold. By that setting, 234 properties are too flat. The others face as follows.

• South48
• South-east47
• East43
• North-east30
• South-west24
• North-west17
• North14
• West9

A south-facing slope gets no official sunshine figure of its own. The solar radiation atlas of the state weather agency (AEMET) gives irradiation for a horizontal surface, on a 3 × 3 km grid with data from 1983-2005, and states that its figures are not directly comparable with inclined surfaces.

The data also leave one question open per property. Which survey the MDT25 rests on at each location is not recorded, so the height accuracy cannot be officially assigned to an individual property.

What should you check on screen, on site and at the town hall?

Most of it can be done before you fly out:

1. Look up the plot's height in Iberpix or in the property report.
2. In SIGNA, draw one longitudinal profile from the access road to the far boundary and a second one across the slope.
3. Read the climate zone for the province and that height in the table annexed to the DB-HE.
4. Check whether a reservoir, a lake or the sea lies close enough to make the model values less reliable.
5. Note the unit of every slope figure you are given, per cent or degrees, before comparing two plots.

On site, look for what no grid shows: fresh banks, retaining walls, damp patches in the ground and the path water takes when it runs off. Then ask two questions. Ask the seller whether a geological or geotechnical report on the plot exists. Ask the ayuntamiento what its planning rules say about building on slopes, and get the answer in writing.

You can start a property check for the plot you are considering.

In short

Spain's terrain model comes from laser surveys and shows the bare ground. Its height error fell from 40 cm in the first survey to 10 cm in the third. Height comes straight from the model, the IGN publishes slope in degrees, and aspect is calculated by whoever states it. A 25-metre cell can swallow a small plot whole, so banks and drops only show on site or in the finer models. Slope is one landslide factor among several in the IGME guide, and the town hall sets the rules for building on a hillside.