> ## Content Index > Fetch the complete content index at: https://blog.topo-grapher.com/llms.txt > Use this file to discover other available public pages before exploring further. # DEM vs DTM: Which Elevation Data Belongs in Revit? - URL: https://blog.topo-grapher.com/dem-versus-dtm-for-accurate-bim-terrain-models/ - Published: 2026-09-17T07:48:05.000Z - Updated: 2026-10-20T19:02:05.000Z - Author: Aki Olafsson "DEM" and "DTM" get used as if they mean the same thing. Sometimes they do. Sometimes the file labelled DEM includes every tree canopy and roof on your site, and the terrain you import into Revit is a few metres too high in exactly the places you care about. For BIM the useful question is not which acronym is correct. It is whether the file represents **bare earth**, at a resolution and coordinate system that suits the decision you are about to make. ## DEM vs DTM: the working distinction A **Digital Elevation Model (DEM)** is the umbrella term for any digital grid of heights. It may describe the ground, or it may describe whatever surface the sensor saw first. A **Digital Terrain Model (DTM)** normally means bare earth: vegetation, buildings and other objects have been classified out, leaving the ground that grading, drainage and finished floor levels relate to. The third term is the **Digital Surface Model (DSM)**, which records the top visible surface: canopy, roofs, bridges. We cover that comparison separately in [DTM vs DSM](https://topo-grapher.com/learn/dtm-vs-dsm.html?ref=blog.topo-grapher.com). For site models the rule is simple: **build terrain from a DTM, and keep any DSM as a separate layer for context.** The labels are not consistent enough to trust on their own. Some agencies call their bare-earth product a DEM; some global "DEMs" are really surface models. Read the dataset description. ## Which well-known datasets are which? | Dataset | What it is | Bare earth? | | -------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------- | --------------------------------------------------------------- | | National LiDAR DTMs (EA LiDAR DTM in England, Kartverket DTM1 in Norway, IGN RGE ALTI in France, USGS 3DEP bare-earth DEM) | LiDAR with ground points classified | Yes | | GEDTM30 (OpenGeoHub, 30 m global) | Global model built specifically as bare-earth terrain | Yes, by design | | Copernicus GLO-30 | Global radar-derived model | No, it is a surface model: forests and dense buildings raise it | | SRTM (30 to 90 m) | Radar data from 2000 | No, close to a surface model in forest and cities | That table explains a common complaint: "the terrain is wrong in the woods". A surface model measured the canopy. On a wooded slope that can mean a false ridge many metres above the real ground. ## Why bare-earth data changes design decisions Terrain stops being a visual task the moment it informs a finished floor level, an accessible route or a drainage direction. Consider a sloping plot with mature trees along one boundary. A surface model rises with the canopy. Import it and the site looks steeper than it is: a path appears non-compliant, or the building platform gets set too high. Around neighbouring buildings, a surface model picks up roofs and creates spikes that look like survey errors. A DTM avoids both. It is not automatically good enough for everything, though. A 30 m national or global DTM is right for a valley-scale masterplan and too coarse to set a threshold level on a small plot. ## Read the data specification before you model Three numbers matter, and they are not the same thing: - **Resolution (grid spacing):** how far apart the heights are. 1 m shows banks, ditches and kerbs; 30 m shows landform. - **Vertical accuracy:** how far a height may be from the truth. A national LiDAR DTM is often around ±0.1 to 0.2 m; global 30 m models are metres. - **Date:** no dataset shows an embankment built after it was flown. Also confirm the horizontal coordinate system and the vertical datum. Mixing ellipsoidal and orthometric heights can shift the whole site vertically by tens of metres in some regions. See the [vertical datum guide](https://blog.topo-grapher.com/vertical-datum-guide-for-bim-terrain-models/). ## How do you get a DTM into Revit? Revit cannot import a GeoTIFF or a raster DEM. It builds Toposolids from points or 3D CAD data. Autodesk lists the accepted inputs as "DWG, DXF, DGN, or CSV points files" ([Autodesk, Create a Toposolid from Imported Data](https://help.autodesk.com/cloudhelp/2026/ENU/Revit-Model/files/GUID-A1E1DE84-29AE-4B99-B377-091C0AC45249.htm?ref=blog.topo-grapher.com)). So a DTM has to become points first: 1. **Get the DTM** for your site from the national portal (or let a terrain service fetch it). 2. **Clip it** to the site plus a margin, and **resample** to a spacing that fits Revit's point limit: 10,000 points on the import page, up to 50,000 from Revit 2026 via the NativeToposolidMaxPointThreshold setting in Revit.ini. 3. **Convert to X, Y, Z points** in metres, comma-delimited, no header. In QGIS that is a raster-to-points step and a CSV export; see [BIM GIS terrain](https://blog.topo-grapher.com/bim-gis-terrain-for-editable-site-models/). 4. **In Revit:** Massing & Site tab > Toposolid > Create from Import > **Create from CSV**, then set the units in the Format dialog. Topo-grapher does steps 1 to 3 in the browser and always uses the bare-earth product where it exists. Outside countries with national LiDAR, it uses GEDTM30, a 30 m global DTM, not a surface model. [Generate a Toposolid-ready CSV free →](https://topo-grapher.com/?ref=blog.topo-grapher.com) ## Choose the right source for the task - **Feasibility and masterplanning:** an official DTM at 1 to 5 m, or 30 m for very large areas. - **Landscape concept:** a DTM, so swales, paths and terraces respond to real ground rather than canopy. - **Detailed grading and construction:** a measured topographic survey, with the DTM as context. - **Height of trees or neighbouring buildings:** a DSM, kept as a separate layer. ## Do a plausibility check after import Compare the terrain with known spot levels, road edges and contours on a map. Look for spikes around buildings, flat patches where the site should fall, and drainage that runs uphill. Those usually mean surface objects in the data, sparse sampling or a coordinate problem, not real terrain. ## FAQ **Is a DEM the same as a DTM?** Not always. DEM is the general term; a DTM is specifically bare earth. Check the dataset description before assuming. **Which one has trees, DTM or DSM?** The DSM. It records the top surface, including canopy and roofs. The DTM removes them. **Is Copernicus GLO-30 a DTM?** No. It is a surface model derived from radar, so forests and dense buildings raise it. GEDTM30 is a 30 m global model built as bare earth. **Can I import a DTM GeoTIFF into Revit?** No. Convert it to a CSV of X, Y, Z points and use Create from CSV. **What resolution DTM do I need for a building plot?** 1 to 2 m shows the features that affect a plot. Sample it at a spacing that keeps the point count within Revit's limit. **Does a DTM replace a topographic survey?** No. It is a strong early-stage input. Construction setting out and legal levels still need a survey. ## Further reading - [DTM vs DSM: what is the difference?](https://topo-grapher.com/learn/dtm-vs-dsm.html?ref=blog.topo-grapher.com) - [Autodesk: Create a Toposolid from Imported Data](https://help.autodesk.com/cloudhelp/2026/ENU/Revit-Model/files/GUID-A1E1DE84-29AE-4B99-B377-091C0AC45249.htm?ref=blog.topo-grapher.com) - [GEDTM30 global bare-earth terrain (OpenGeoHub, PeerJ 2025)](https://doi.org/10.7717/peerj.19673?ref=blog.topo-grapher.com) - [USGS 3D Elevation Program](https://www.usgs.gov/3d-elevation-program?ref=blog.topo-grapher.com) - [Elevation data coverage by country](https://topo-grapher.com/elevation-data-coverage.html?ref=blog.topo-grapher.com)