Open any topographic survey drawing and the first thing you see is a set of curving, roughly parallel lines. Those are contour lines. Anyone who can read them sees at once where the hilltops are, where water runs, which slopes are steep and which are gentle.
This article has two parts. The first explains what contour lines are and how to read a contour map. The second shows how to build them in Civil 3D from a file of survey points, in five steps with a screenshot of every dialog. The data set used here is attached so you can download it and follow along.
What are contour lines
A contour line joins points of equal elevation on the ground. Imagine slicing a hill with horizontal planes one metre apart. Where each plane cuts the hill, projected down onto the drawing, you get one line.
The height difference between two neighbouring lines is the contour interval. With a 1 m interval, moving from one line to the next takes you exactly one metre up or down.
Maps usually show two kinds of line:
- Major contours: every few lines, one is drawn heavier and usually carries its elevation. In the image below these are the green lines, 5 m apart.
- Minor contours: the thin lines between two major ones, 1 m apart.
These lines have four properties worth remembering:
- Every point on one line has the same elevation.
- Lines never cross. The only exceptions are vertical cliffs and overhangs.
- Each line is a closed loop, even if it closes outside the map.
- The closer the lines, the steeper the ground. The wider apart, the gentler.
How to read a contour map: summits, saddles, valleys
The shapes of the lines alone reveal the basic landforms. The image below is the right half of the site used in this article.
| No. | Landform | How it shows on the map |
|---|---|---|
| 1 | Summit | Closed loops nested inside each other, rising towards the centre. The innermost loop here is 37 m |
| 2 | Saddle | The lowest point between two summits. Two sets of loops turn their backs on each other with a gap between |
| 3 | Valley | Lines bend into a U or V. The point of the V aims uphill, against the direction water flows |
| 4 | Gentle slope | Lines spaced far apart |
A ridge looks like a valley turned around: its V points downhill. To tell them apart, read the elevations on two neighbouring lines.
Slope is i = Δh / d, where Δh is the contour interval and d the horizontal distance between two lines measured on the map. With a 1 m interval and lines 5 m apart, i = 1/5 = 20%. On the same hillside, where the lines are 20 m apart the slope is only 5%.
The elevation of a point between two lines is found by interpolation. A point halfway between the 30 m and 31 m lines sits at about 30.5 m.
Preparing the survey point file
To learn how to draw contour lines, start with data. The lines are interpolated from elevation points measured on site. A total station or GNSS receiver exports them as a text file, one point per line.
The data set here holds 917 points on an imaginary 400 × 300 m site. The file is in PNEZD order, comma-separated.
| Column | Code | Example | Meaning |
|---|---|---|---|
| 1 | P (Point number) | 1 | Point number |
| 2 | N (Northing) | 2002.803 | North coordinate, i.e. Y |
| 3 | E (Easting) | 1001.312 | East coordinate, i.e. X |
| 4 | Z (Elevation) | 18.097 | Elevation in metres |
| 5 | D (Description) | DTN | Description: DTN is natural ground, SUOI a stream bed point, DINH a summit |
Column order matters more than you might think. Many survey teams put Y before X. Pick the wrong format on import and the points land in the wrong place, as the common mistakes section shows. Handling survey data is also the first session of the Institute's online Civil 3D course.
Step 1: Import the points into Civil 3D
Start a new drawing from the _Autodesk Civil 3D (Metric) NCS.dwt template so metric units and display styles are ready. Then open the import dialog.
| No. | Item | Meaning |
|---|---|---|
| 1 | Insert tab | The tab holding commands that bring data into the drawing |
| 2 | Points from File | Opens the Import Points dialog. Typing IMPORTPOINTS does the same |
In Import Points, set the file, the format and the point group in turn.
| No. | Item | Meaning |
|---|---|---|
| 1 | + button | Browse for the point file |
| 2 | Selected Files | The chosen file. A green tick means it matches the selected format |
| 3 | Specify point file format | Choose PNEZD (comma delimited) to match the file's column order |
| 4 | Preview | Check a few rows. Point Number should read 1, 2, 3 and Point Elevation should hold elevations |
| 5 | Add Points to Point Group | Tick it and choose a group to collect the imported points |
| 6 | OK | Import |
If the group does not exist yet, click the button to the right of the group box (item 5). A small dialog asks for the name.
| No. | Item | Meaning |
|---|---|---|
| 1 | Group name | Type a name such as DIA HINH TU NHIEN. If it already exists, Civil 3D reuses it |
| 2 | OK | Create the group and return to Import Points |
Click OK in Import Points. All 917 points appear. Zoom in and each point shows three lines of text: number, elevation and description.
Step 2: Create the terrain surface
In Civil 3D these lines are not drawn separately. They are one way of displaying a surface. So the surface comes first.
| No. | Item | Meaning |
|---|---|---|
| 1 | Surfaces | Drop-down of surface creation commands |
| 2 | Create Surface | Creates a new, empty surface to add data to later |
| No. | Item | Meaning |
|---|---|---|
| 1 | Type | Keep TIN surface: triangles joining the survey points |
| 2 | Name | Surface name, e.g. DIA HINH TU NHIEN |
| 3 | Style | Display style. Choose Contours 1m and 5m (Design) |
| 4 | OK | Create the surface |
Click the Style cell, click the ellipsis button, then pick a style from the list. The style name gives the interval: 1m and 5m means minor lines every 1 m and major lines every 5 m.
This site rises about 20 m, so 1 m / 5 m suits it. Hilly sites spanning hundreds of metres call for 5 m / 25 m, so the drawing does not fill up with lines.
Step 3: Add the point group to the surface
The new surface is still empty. You must tell it where its elevations come from. Open Toolspace, Prospector tab, and expand the tree.
| No. | Item | Meaning |
|---|---|---|
| 1 | DIA HINH TU NHIEN | The new surface, under Surfaces |
| 2 | Definition | The surface's data sources: boundaries, breaklines, existing contours, DEM files, point groups... |
| 3 | Point Groups | Where point groups are added to the surface |
Right-click Point Groups and choose Add.
| No. | Item | Meaning |
|---|---|---|
| 1 | Point Groups | The Definition node that takes point groups |
| 2 | Add... | Opens the group picker |
| No. | Item | Meaning |
|---|---|---|
| 1 | DIA HINH TU NHIEN | The group holding the 917 imported points. Avoid _All Points: design points added later would be pulled into the surface too |
| 2 | OK | Build the surface |
Click OK and the lines appear straight away. Point labels now cover most of the drawing. Turn off the V-NODE (point markers) and V-NODE-TEXT (label text) layers in the Layer list to see the map clearly, as in the first image.
Step 4: Change the interval and label elevations
To change the contour interval, right-click the surface name in Prospector and choose Edit Surface Style.
| No. | Item | Meaning |
|---|---|---|
| 1 | Surface name | Right-click here |
| 2 | Edit Surface Style... | Opens the style editor |
Go to the Contours tab and expand Contour Intervals.
| No. | Item | Meaning |
|---|---|---|
| 1 | Contour Intervals | Base Elevation is the datum, Minor Interval the minor contour spacing, Major Interval the major spacing |
| 2 | OK | Apply |
Editing here changes the style, so every surface using it changes too. To change one surface only, pick another style in step 2, or copy the style before editing it.
To label elevations, go to the Annotate tab, click Add Labels, choose Surface, then Contour - Multiple.
| No. | Item | Meaning |
|---|---|---|
| 1 | Add Labels | Label menu on the Annotate tab |
| 2 | Surface | Surface labels |
| 3 | Contour - Multiple | Labels every line a straight segment crosses |
Pick the start and end of a segment crossing the lines, then press Enter. Each crossing gets an elevation label. Repeat on other slopes.
| No. | Item | Meaning |
|---|---|---|
| 1 | Label segment | Drag from outside towards the summit. Labels sit where the segment crosses each line |
| 2 | Command line | ADDCONTOURLABELING asking for the next point |
Labels are tied to the surface. Add survey points or edit the surface and the Civil 3D contour lines redraw, with label values updating themselves. That is where Civil 3D differs from drawing by hand in AutoCAD. Label options are described in the Autodesk Civil 3D help.
Common mistakes when creating contours
Choosing the wrong point file format. This is the most common one. Import Points often has ENZ preselected, meaning the first column is Easting. With a PNEZD file, the point number is read as X and the X coordinate as elevation.
| No. | Item | Meaning |
|---|---|---|
| 1 | ENZ (comma delimited) | The wrongly selected format |
| 2 | Preview | Elevations of 1001.312, dozens of times the site's relief: a sure sign the columns are shifted |
Always check Preview before clicking OK. The green tick under Selected Files only says the column count fits, not that the order is right.
Empty surface, no lines. Usually step 3 was skipped. After creating the surface, add the point group to its Definition.
Lines packed solid. The interval is too small for the relief. Pick 2 m / 10 m or 5 m / 25 m.
Lines running outside the survey area. The TIN joins points far apart along the edge, creating long thin triangles and odd curves. Fix it by adding a boundary under Boundaries in the Definition.
Frequently asked questions
What contour interval should I use?
It depends on map scale and terrain. Flat ground needs a small interval, 0.5 m or 1 m, to show any relief. Hilly ground uses 2 m, 5 m or more. The rule is that lines must never be so tight you cannot tell them apart.
What is the difference between major and minor contours?
Major lines are heavier, repeat after a set number of minor lines and usually carry labels. With a 1 m / 5 m style, every fifth line is major.
Do I need Civil 3D to produce a topographic map?
No. You can interpolate by hand or use a lisp routine in AutoCAD. But then the lines are just linework, and changing the data means redrawing. In Civil 3D they belong to the surface and update themselves, and the same surface goes on to profiles, cross sections and earthwork quantities.
Where can I get Civil 3D?
Students and lecturers can use the Education version free of charge. Installation is covered in Install Civil 3D free for students.
Related articles
The terrain surface is the first link in the surface, alignment, profile, corridor chain described in What is Civil 3D. Revit users can compare it with building terrain as a Toposolid in Creating terrain in Revit.
To work through the whole path from survey data to a road design package, the Institute's online Civil 3D course devotes its first two sessions to survey points and terrain surfaces.