Updated: Feb 23, 2022
Topographic maps are just one of the maps your land developer will create. They can be confusing to decipher, especially if you’re not used to seeing elevation laid out on a 2D plane. Once you understand how to read a topographic map, however, it will become a breeze.
What is a Topographic Map?
A topographic map is unlike any other map. You’re probably most familiar with seeing them outline hiking trails, though they’re commonly used in construction and development all over the world.
The main purpose of topographic maps is to give a birds-eye view of the landscape to show elevation and distinct natural features, such as rivers, valleys, and mountains that are carved into the Earth’s surface. Each of these changes in elevation are represented by lines called contour lines.
These contours, which connect points of the same elevation, make it possible to show the height and shape of hills and mountains, and the depths of oceans, rivers, lakes, and more.
Some topographic maps also show many other kinds of geographic features including roads, railroads, rivers, streams, lakes, boundaries, place or feature names, mountains, and much more.
How Are They Used for Land Development?
Conducting a topographical survey is a vital step in choosing an appropriate piece of land for your custom home. It will give you a picture of what the land currently looks like, including the presence of trees and the boundaries of the property. It will also allow you to see what kind of work may need to be done to facilitate safe construction.
Many buyers assume they should always choose the flattest and driest land possible, but slopes, rivers and streams, and other bodies of water can often be worked into your design plans in creative and functional ways.
On top of that, a topographic map helps developers and builders see the full picture, such as placements for your home, pools, fencing, driveways, and more. It can also show existing residential development, such as
Roads and highways
So, how do you read a topographic map?
How Contour Lines Describe Terrain and Elevation
Contour lines are fairly simple to understand; they’re not used for navigation, but rather to give shape to a change in elevation on a 2D plane. Contour lines also indicate the shape of the terrain. Roughly concentric circles are probably showing you a peak, and areas between peaks are passes. Studying a topo map of a familiar area is a great way to learn how to match terrain features with the contour lines on a map.
Let’s use this picture as an example:
As you can see, some lines are marked with numbers, which represent the elevation level along each line. Do you see the tiny circle in the middle of the picture, just to the left of the 5030m mark? That is a mountain peak. Knowing that, follow each line down to know how steep the mountain is, as well as how flat the area around it is. The closer the contours are together, the quicker the change in elevation.
Now look to the bottom left corner of the image. Notice how the 4880m contour outlines a larger, oblong shape? That means the oblong shape is relatively flat at 4880m in elevation. Follow that contour to the right and you’ll also notice that the 4880 elevations rapidly ascends or descends as you get closer to the middle of the picture. Understanding the change in steepness will help you understand where to build, and where not to build.
Other maps may include blue lines to represent rivers and waterways, or dotted lines to represent trails.
You may also notice that some contour lines are bolder than others. That simply means those lines have the elevation listed somewhere along them. They’re referred to as “index contour lines” and appear at every fifth line.
What About Measuring the Change of Elevation?
All topographic maps are easy to read because they have a set “contour interval,” meaning you know that every line increases by X feet/meters in elevation. To use the above picture as an example, every contour line represents an increase/decrease of 10m.
Many maps have either a 40- or 80-foot contour interval. You find the contour interval for your map in its legend.
How to Read a Topographic Map Scale
Topographic map scales are no different than that of a regular map. Their scale tells you how detailed the map is. For example, a 1:24000 scale means one inch on the map equals 24,000 inches of real-world terrain.
This is where it can be confusing. When a topographic map has a “large scale,” it means the map has more detail. When it has a “small scale,” such as that of 1/65,000, the map is less detailed.
Maps also have a representative scale to help you visualize real-world distances. You can use this scale and a string or the edge of your compass to get a rough estimate about how far your planned building will be from the road, river, or other feature.
Other Useful Topographic Map Details
Your map’s legend should be the first thing you familiar yourself with. It will detail what color contour line means what, if there are any special areas of interest marked by a symbol, as well as the map’s scale.
For example, a blue line usually indicates there is a river, a large green spot usually means thicker vegetation in that area, and light greens on the map usually indicate the area is relatively clear. When developing a home, these colors are critical to knowing how rainwater will impact the area, if you have to worry about grading the land, and how to install utilities such as sewer lines and underground cables. It will also let you know if you will need to construct a bridge for transport, or if you will need to worry about the possibility of your future building being in a floodplain.
That’s it! Topographic maps are extremely useful tools for land development, and as the land owner, knowing how to read one is imperative to help you understand your developer’s actions, as well as how the land could impact future construction plans.