Land records · Parcel maps · October 7, 2026
Contour Lines on a Map: Reading Elevation
How to read contour lines on a map: lines connecting equal elevations, contour intervals, slope steepness, and how contours appear on plats and GIS.
Contour lines on a map connect points of equal elevation, showing the shape of the land on a flat drawing. Each line represents a specific height above a reference datum, and the spacing between lines shows steepness: close lines mean steep slopes, while widely spaced lines mean gentle terrain. Contours appear on topographic maps, grading plans, site plans, and GIS layers derived from survey or lidar elevation data. Reading contours helps owners understand drainage, slopes, and building sites. This page is general land-records background, not legal advice.
Key terms
| Term | Meaning |
|---|---|
| Contour interval | Elevation difference between lines |
| Index contour | Bolder line, labeled with elevation |
| Intermediate contour | Lighter lines between index contours |
| Depression contour | Hachured line showing a low spot |
Reading slope
Close contours indicate steep slopes, important for steep slope ordinances. To estimate slope, divide the contour interval by the horizontal distance between lines and multiply by 100.
Reading drainage
Water flows perpendicular to contours, downhill. Contours forming a V pointing uphill indicate a valley or stream. A V pointing downhill indicates a ridge.
Contours on plans
Grading plans show existing contours, often dashed, and proposed contours, often solid, to illustrate earthwork.
Contours in GIS
County viewers often provide contour layers at intervals like 2 or 10 feet. See how to use a county GIS viewer.
Accuracy
Contours generated from lidar or older maps are approximate. Engineering work requires a topographic survey.
Example
A parcel map shows 2-foot contours. Near the road, lines are 40 feet apart, indicating a 5 percent slope suitable for a house. Toward the rear, lines are 8 feet apart, a 25 percent slope, which local rules restrict.
Common mistakes
Confusing contour intervals between maps, ignoring the datum, and assuming contours are exact can lead to errors.
Contours and flood risk
Contours can help visualize low areas, but official flood status comes from FEMA maps and surveys.
Contours and building sites
When choosing a house site, look for areas where contours are spaced widely, indicating flatter ground. Check that water from uphill can be directed around the site rather than toward it. Contours bending toward the site from uphill suggest a natural drainage path that could bring runoff to a foundation. A driveway route should cross contours at a gentle angle to keep grades manageable.
Contours on older maps
USGS topographic quadrangles from the mid-1900s used contour intervals like 10 or 20 feet. Their elevations may be based on older vertical datums, such as NGVD29, so values may differ slightly from modern lidar contours based on NAVD88.
Spot elevations
Maps may also show spot elevations, single points labeled with heights, such as hilltops, road intersections, or benchmarks. Spot elevations help fill in details between contour lines.
Bottom line
Contour lines connect equal elevations, showing terrain shape, steepness, and drainage. Close lines mean steep slopes, and V shapes show valleys and ridges. Use contour layers for planning, and rely on surveys for design. Explore terrain via the Platbookmapper map.
Contour lines on a map FAQ
What are contour lines?
Lines connecting points of equal elevation.
What is a contour interval?
The elevation difference between adjacent lines.
What do close contour lines mean?
Steep terrain.
How do I find contours for my property?
Check county GIS or USGS maps.
Are GIS contours accurate enough for building?
No, use a topographic survey for design.