Slope is a simple calculation of a vertical distance divided by a horizontal distance (with both distances in the same measurement units) that is often expressed as a percentage.
In the area of land development, a small ground slope would be something that is one percent (1-foot elevation change over a 100-foot horizontal change) or less. A large ground slope would be something that is greater than fifty percent (50-foot elevation change over a 100-foot horizontal change). The slope of a part of your property is a piece of information that can be obtained through a couple of means.
The following are seven ways to find the slope of a part of your land.
1) Use a local GIS.
2) Look at a topographic map.
3) Review an existing conditions plan.
4) Review a past land development plan.
5) Use a soils map.
6) Measure the slope yourself.
7) Hire a surveyor.
1. Use a Local GIS
If you live in a county that has an online GIS program with public access, you could use the maps within this website to find the slopes of different parts of your property.
Once you locate your property, you could use an online measuring tool that should be a part of the GIS website to measure the distance between two points on your parcel. You could then use the contours on the maps to read the difference in elevations between the two points. You would then have the numbers required to calculate the slope between the two points.
2. Look at a Topographic Map
Through an online service (such as the one provided by the United States Geological Survey), a topographic map can be used to figure out a slope by measuring horizontal distances between two points within the website and then reading the corresponding elevations to figure out the change in elevations.
If you have a hard copy plot of a topographic map, you would have to use an engineering scale to measure the distances off of the map. You would still read off elevations from contours to figure out elevation differences.
The slope between the two points you pick from the topographic map would be the elevation difference divided by the horizontal distance.
3. Review an Existing Conditions Plan
If you have access to an existing conditions plan for your site, you would have access to the information required to determine the slope for certain areas of your land.
An existing conditions plan is a plan that you or a previous property owner might have had prepared for your site by a land surveyor. You could measure horizontal distances and read elevations off of contours to obtain the numbers required to calculate the slope between two points.
An existing conditions plan might even already have certain areas of your site labeled with ground slopes.
4. Review a Past Land Development Plan
A past land development design plan either for your individual property or for your entire neighborhood would also contain the information necessary to calculate land slopes. You would have to measure horizontal distances using an engineering scale and then read elevations off of existing or proposed contours to get the numbers required for calculating the slope between two points.
Even if you have only a digital copy of a past plan, you could still estimate distances using the graphic scale that should be included on the plan.
5. Use a Soils Map
The soils map service provided by the Natural Resources Conservation Service could be used to determine general slopes for your property and surrounding areas. You would have to locate your property and then download a soil report to review. Within this report you should be able to determine the type of soils on which your site is located along with the corresponding land slopes.
Keep in mind that this method of finding slopes should be used if you are interested in determining general slopes or ranges of land slopes. One of the other methods in this post would be a better option if you want a more accurate slope for a specific area of land.
6. Measure the Slope Yourself
If you want to know the slope of a specific part of your property, you could measure it yourself.
You would start by taking a standard 12-inch ruler or a 12-inch engineering scale and holding it level with one end against the ground for which you are measuring the slope. Then you would have to measure the vertical distance (in inches) of the other end of the ruler or engineering scale from the ground.
You could then calculate the land slope by dividing the measured vertical distance by 12 inches (the horizontal distance of the ruler or engineering scale).
7. Hire a Surveyor
Although it would be an expensive option, hiring a surveyor to create an existing conditions plan for your property could be a good idea if you plan on developing anything on your property soon anyway.
You could use the existing conditions plan to do your own land slope calculations or you could request that the land surveyor (or whoever it is that actually prepares your plan) add extra labels to your plan where you want to see the slopes after they are calculated.
Finding the Slope of Land Is Not Difficult
As you can see, finding the slope of an area of land is not difficult. It just takes two numbers. Once you determine the change in vertical distance and the change in horizontal distance, calculating the slope is just one step away.
What can be a little more difficult is figuring out those changes in distance. The methods for finding these changes presented above can be good options to consider.