Elevation refers to the vertical distance of a point on the Earth’s surface relative to a fixed, zero-reference plane. Calculating the elevation difference between two points, often called the “rise,” is a common task across many fields. This vertical separation is important for construction planning, ensuring proper water drainage, and mapping the difficulty of a hiking trail. This measurement helps users understand the true slope and contour of the land.
Fundamental Principles of Elevation Difference
Elevation is calculated in relation to a defined reference surface known as a vertical datum. The most common datum is the geoid, which approximates the average sea level across the globe, referred to as orthometric height. For any calculation, the first step is to obtain the elevation of the starting point (Point A) and the ending point (Point B) relative to this common datum.
The core mathematical principle is straightforward subtraction: the elevation difference is the elevation of Point B minus the elevation of Point A. A positive result indicates a net gain in elevation, while a negative result shows a drop in vertical distance. This vertical difference, combined with the horizontal distance between the two points, defines the terrain’s slope or grade. The slope is expressed as the ratio of the rise (vertical difference) over the run (horizontal distance).
Calculating Elevation Using Digital Resources
The fastest and most accessible method for finding elevation data involves using widely available digital mapping tools. Services like Google Earth or specialized online viewers utilize global digital elevation models (DEMs). These DEMs are large datasets containing elevation readings for the entire planet, often derived from satellite or aerial radar missions.
To use these tools, a user places a marker on a map at the first location, and the software instantly returns the estimated elevation for that coordinate. Repeating this action for the second point provides the two necessary values for subtraction. This method is convenient and offers accuracy sufficient for general planning, such as visualizing a route profile.
Modern Global Positioning System (GPS) devices and smartphone applications also provide instantaneous elevation readings. While GPS is highly accurate for horizontal position, its vertical accuracy is inherently lower, sometimes being up to two times less precise due to satellite geometry.
Barometric altimeters offer a more reliable local elevation reading by measuring atmospheric pressure, which decreases predictably with altitude. However, these readings must be calibrated frequently, as changes in weather-related air pressure can introduce errors. It is important to remember that digital elevation data is often an estimate based on a terrain model, not a true survey measurement.
Direct Measurement Techniques and Tools
When high accuracy is required, such as in landscaping or small construction projects, direct, on-site measurement techniques are necessary. A simple, low-cost method for short distances involves using a water level or a long spirit level mounted on a straight board. By setting up a fixed reference post at the starting point, the elevation can be “stepped off” incrementally to a second post.
This technique uses the horizontal plane established by the level to measure the vertical difference over a short distance. The vertical distance from the level line to the ground is measured at each post. The difference between these measurements yields the elevation change for that segment. This process is repeated until the full distance is covered, with all the small vertical differences added together.
For the highest degree of accuracy, professional surveyors use differential leveling with an optical instrument like an automatic level or transit level. This process involves setting up the instrument between two points and using a graduated measuring rod. The surveyor first takes a “backsight” reading on a point of known elevation. This reading is added to that point’s elevation to calculate the Height of the Instrument (HI).
The instrument is then rotated to take a “foresight” reading on the unknown point. Subtracting the foresight measurement from the calculated HI provides the precise elevation of the new point. This systematic method of transferring elevation is the standard for engineering and construction, providing measurements accurate to within millimeters.