Lähtötasot

Lähtötasot are reference elevations used in surveying, construction, and cartography as the starting point for vertical measurements. They form a local or project-specific vertical datum, providing a common baseline from which heights, depths, and terrain elevations are calculated. By defining a stable set of starting levels, different datasets and drawings can be aligned and compared consistently.

Establishing lähtötasot typically involves a leveling survey to create a network of benchmarks with known elevations relative to a national vertical datum. This national datum is usually tied to mean sea level or a geoid model and is updated as geodetic models improve. In practice, several lähtötasot may exist for different projects, regions, or time periods, reflecting changes in measurement methods, datum definitions, or local conditions.

In daily practice, elevations in plans and digital models are expressed as values above the chosen lähtötaso. For example, a floor level or a point on a construction site may be stated as a certain number of meters above the project datum, and all other elevations are derived from that reference.

In Finland and other countries with established surveying traditions, vertical reference systems are maintained by national authorities and account for factors such as sea-level changes, land uplift, and geoid refinements. The concept of lähtötasot enables accurate planning, flood risk assessment, and infrastructure design by providing a stable basis for vertical measurements across maps, models, and records.

See also: vertical datum, leveling, benchmarks, geodesy.