Contours or DEM? Choosing between vector contours and raster elevation for site set-up

There is a moment early on almost every site set-up where the same question comes up: do you pull down contours, or do you pull down the underlying elevation grid? Both describe the same terrain. Both come from the same LiDAR. But they behave very differently once they land in your software, and picking the wrong one costs you an afternoon of remodelling.

This is a comparison worth getting right, because the two formats are not interchangeable. A DXF full of contour polylines and a GeoTIFF DEM (Digital Elevation Model) each have jobs they do well and jobs they do badly. Here is how they stack up, and which one to reach for depending on where the data is going.

The two things you can download

When you export elevation data for a QLD, NSW or Victorian site, you are usually choosing between two representations of the best government elevation surface available for that location. Resolution, capture method and currency vary by state and area, so do not assume every address has the same LiDAR coverage.

Vector contours are lines of equal elevation, delivered as polylines in DXF, or as features in a Shapefile or GeoPackage. Each line carries an elevation value. They are a derived product: someone chose a contour interval and generated lines from a grid.

A DEM raster is the grid itself: a GeoTIFF where every cell holds a height value. It is closer to the source surface, and contours can be generated from it. Grid resolution varies with the government source available for the selected site.

The distinction matters because a contour set has already had a decision baked into it. If you download 0.5 m contours you cannot get 0.25 m detail back out of them. A DEM keeps your options open.

Where each one shines

Contours are immediately readable. Drop them into a drawing and an engineer or surveyor reads the terrain at a glance: ridges, gullies, batters, the fall across the pad. They plot cleanly, they annotate cleanly, and they are what a set of drawings ultimately needs to show.

A DEM is the better raw material. It builds a surface (a TIN or grid) in Civil 3D or QGIS without you tracing anything, drives cut and fill volumes, generates slope and aspect analysis, and lets you re-contour at any interval you like after the fact. If you are doing genuine terrain analysis rather than just drawing lines, the raster wins.

The catch with a DEM is bulk. A 1 m raster over a large site or a whole estate is a heavy file, and older CAD workflows are happier with vectors than rasters. The catch with contours is the opposite: they are light and tidy, but they have thrown away information, and their usefulness is locked to the interval someone already chose. If that interval is wrong for your job, you are stuck.

Contours vs DEM raster, side by side

Axis	Vector contours (DXF / SHP / GPKG)	DEM raster (GeoTIFF)
What it is	Lines of equal height, one interval baked in	Full elevation grid, every cell a height
Best for	Drawings, quick terrain read, annotation	Surface building, volumes, slope analysis
Re-interval later	No, locked to what you downloaded	Yes, re-contour at any interval
Detail retained	Only what the interval captures	Full grid resolution (often 1 m)
File size	Light	Heavy over large areas
Civil 3D	Import polylines, add elevation as feature lines	Import as surface directly
QGIS / ArcGIS	Loads as vector layer	Loads as raster, drives analysis tools
Coordinate handling	Needs correct CRS on import	Georeferenced, but check datum

Which should you pick

For most early site set-up and desktop review, download the DEM raster and generate contours from it in your own software. It gives you the surface for volumes and slope, and you control the contour interval instead of inheriting someone else's. This is the flexible starting point, and it future-proofs the job when the design scope shifts.

There is one honest exception. If all you need is a quick visual read of the fall across a block, for a feasibility sketch or a listing review, and you are working in a vector-only workflow, grab the contours at a sensible interval and move on. It is faster, lighter, and enough. Deciding what interval counts as sensible is its own small skill, and choosing the right contour interval is worth a read before you commit.

One more thing that will save you grief regardless of which you pick: sort out the datum and projection at import, not later. Elevation values are tied to AHD (Australian Height Datum), and horizontal positions to MGA2020 or GDA94, and a surface that lands in the wrong place is a surface you will rebuild. Getting the coordinate system right for QLD and NSW site data up front removes the single most common cause of an elevation set that will not line up with the cadastre.

What the current export supplies

The live LayeredGeo catalogue offers contours, elevation points, a GeoTIFF elevation raster and a DXF TIN for supported QLD, NSW and Victorian sites. Contours can be written to DXF, Shapefile, GeoPackage or KMZ; the raster remains a GeoTIFF and the TIN is DXF-only. The interval and source resolution vary by state and location.

The current sample export shows the distinction in a real package: 14 contour features, 500 sampled elevation points, a GeoTIFF DEM and a 2,999-face DXF TIN. Its DATASETS.txt manifest records each requested layer and whether it contained data, was assessed with no features, lacked published coverage, or failed upstream.

Common questions

Can I convert a DEM to contours in Civil 3D or QGIS?

Yes, and it is the recommended path. In Civil 3D you build a surface from the raster, then generate contours at whatever interval and smoothing you want. In QGIS the Raster to Contour tool does it in one step. Going the other way, contours back to a good DEM, is far messier and loses detail, which is why downloading the grid gives you more room to move.

What DEM resolution do I get for QLD and NSW sites?

Some populated areas have fine LiDAR-derived elevation coverage, while regional or older captures can be coarser. Coverage, resolution and vintage vary, so confirm the source for the specific address before relying on it for volumes.

Is a DEM accurate enough to design footings or drainage?

It is excellent for early design, feasibility and desktop review, but it is a surface model, not a survey pickup. For detailed design and setout you still want a ground survey to confirm levels and features the LiDAR cannot see under vegetation or structures. Treat the DEM as your fast, reliable starting surface, then verify on the ground where the design demands it.

When you want the elevation grid and contours prepared together, check the current datasets for your address, review the live dataset catalogue, or inspect the 17-dataset sample ZIP before buying.

LayeredGeo clips QLD, NSW and Victorian site data into supported GIS, CAD, terrain and imagery outputs. Published mapping supports desktop and early-design work; it does not replace a site survey.

About this article. Published by LayeredGeo. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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