In surveying, what are the two horizontal crosshairs used for?

The two horizontal crosshairs in surveying are essential for determining horizontal distance when positioned on a slope. When a surveyor looks through the instrument, these crosshairs enable them to align properly with a point of interest at the same elevation, which is crucial for measuring distances accurately in a sloped terrain.

When surveying on slopes, it is vital to ensure that the distances measured reflect the true horizontal distance rather than the inclined distance. The crosshairs assist in maintaining a level line of sight, enabling the surveyor to read measurements that can be adjusted to account for the slope. This technique helps in providing more accurate data for mapping and construction purposes.

The other options relate to different aspects of surveying that do not utilize the two horizontal crosshairs in the same way. For instance, calculating elevation and measuring vertical angles often involves other tools or methods. Setting property boundaries relies more on overall measurements and corner definitions than the specific use of crosshairs alone.