Line of Sight and Obstacle Clearance

Sight line elevation

First add the eye, instrument or target height to each ground elevation. Join these two upper points with a straight line. Linear interpolation using the horizontal distance ratio gives the line elevation at the obstacle position. Subtract the obstacle top elevation to find vertical clearance.

Example and limits

Endpoint elevations of 102 and 112 m give a line elevation of 107 m at the midpoint. An obstacle top at 105 m leaves 2 m of vertical clearance. This checks only one point. A tree or structure elsewhere may still block the view. Earth curvature and atmospheric refraction are excluded, so the result cannot certify long distance visibility, road safety or radio link performance.

Check the intermediate point on a common elevation datum

With starting ground at 120 m and eye height of 1.5 m, the first endpoint is at 121.5 m. End ground at 128 m and target height of 0.5 m place the other endpoint at 128.5 m. Choose horizontal separation of 80 m and obstacle distance from the start of 20 m. Line elevation is 121.5 + (128.5 − 121.5) × 20 / 80 = 123.25 m. An obstacle top at 123 m gives vertical clearance of 123.25 − 123 = 0.25 m. This example demonstrates linear calculation steps rather than a surveyed route.

Ground elevation and obstacle height above its base are different inputs. If only obstacle height is known, first add it to ground elevation at that location. All elevations must share a common reference. Distance fields use horizontal distances, not the inclined line length. Clearance is vertical, not the shortest distance perpendicular to the line. For several obstacles, check each position and top elevation separately. A small positive clearance is not a safety allowance for measurement error. The calculator does not determine survey uncertainty, unmeasured terrain sections or additional space needed for visibility.

Frequently asked questions

How do you calculate the line of sight between two points?

Linear interpolation gives a sight line elevation of 106.000 m and clearance of 2.000 m above the 104 m obstacle at 40 m along a 100 m span. This preliminary example uses ground elevations 100 and 110 m with endpoint heights 2 and 2 m, excluding Earth curvature and other obstacles.

Calculate with these values