A lot of research and work goes into making sure that a land survey is accurate. Land surveyors have to take the time to locate property boundaries, accurately mark features on the land (including structures, trees, roads, and more), and make sure everything is scaled correctly on the final land survey map. Perhaps the biggest challenge of preparing an accurate land survey, though, is mapping out the topography of the terrain. Since land surveys are sketched two-dimensionally on paper, land surveyors need a way to show variations in elevation. The answer is contour lines, which are a vital part of any topographic land survey.
What do contour intervals on topographic map show?
Contour intervals on a topographic map are meant to show lines of elevation. Each contour line shows a different elevation. That way, the survey can show a hill or valley even within the constraints of a two-dimensional map. The lines are drawn to show where the relief of the land is changing.
A contour interval is not the same thing as a contour line. A contour line is the actual marking that a surveyor draws on a topographic map to represent a specific elevation. Contour lines can theoretically be drawn for any elevation. However, you can’t have a contour line for every single shift in level. Most slopes are gradual, whether they are going upward or downward. As a result, when it comes to showing varying elevations on a land survey, you can’t draw a contour line for every inch (or even every foot) of variation. With too many contour lines, your map could quickly become difficult or downright impossible to read.
That’s where contour intervals come into play. A contour interval refers to the vertical distance between contour lines and the elevation they represent. Say you have a hill that needs to be marked on a map. The highest elevation of the hill is 800 feet above sea level. The lowest elevation is 700 feet. The hill slopes up gradually between those two elevations. You would obviously have contour lines at the elevations of 700 and 800 feet, but you would also want to account for all the space in between those raises.
Depending on how many horizontal inches of the map are used to show the hill, you would change the contour interval to make sure that each individual contour line was clearly legible. So you might draw in additional contour lines for every 10 feet, 5 feet, or 2 feet of vertical elevation for a hill like this. In other words, every time the hill gains elevation equal to the contour interval, you’d draw another contour line on the map. In such a scenario, the contour interval would likely be 10 feet. These lines would make it possible to visualize the hill—and estimate the elevation at certain points—just from looking at the map, without having so many lines that it was hard to read.
Choosing the Right Contour Intervals
Not all topographic maps have the same contour intervals. Because contour intervals on topographic maps show changes in elevation, it stands to reason that what is the “right” contour interval for a map will vary depending on the terrain being surveyed. A flat site without any major changes in elevation might require a 1 foot contour to represent the elevation shifts since this would keep the distance between the contours from getting too large which would make the map difficult to read.
The nature of the terrain isn’t all that will impact how contour lines are drawn on the map. The purpose of the survey—and the level of accuracy and detail required—are also important determinant factors. In some cases, like when planning cut and fill for a project, you might need a smaller contour interval to plot out an effective excavation plan. Having more contour lines—and likely smaller contour intervals—would give you a more exact idea of the elevation level at any given point. A more general purpose land survey may not demand that level of detail and would include fewer contour lines and larger contour intervals.
Do keep in mind that a map with wider contour intervals and fewer contour lines is less precise than one with tighter intervals and more lines. The tradeoff for a smaller contour interval is an increase in cost since most Land Surveyors estimate projects based on the time it will take to gather all of the information and draft the map. A topographic map with more precise elevation readings would take longer to prepare. As a result, that type of survey would likely cost you more money than a simpler survey with larger contour intervals. In addition, a map with more details will also give you a greater level of accuracy.
Call Hogan Land Services for Help with Your Topographic Land Survey
At Hogan Land Services, we offer topographic land surveys in all Bay Area counties from our offices in Santa Rosa, Santa Cruz and Livermore. We can also advise you on the level of detail you want out of your survey based on its intended purpose. Give us a call today to learn more about our surveys and find out what contour intervals are most appropriate for your topo map. You can reach us by calling 877.544.2104 or filling out the contact form on our website.