Surveying is the technique and profession of determining the boundaries of land.
First, benchmarks are located at known latitude and longitude. They are far enough apart from each other that spherical geometry is not accurate enough to compute their distances, and a geoid must be used. The ancestor of all benchmarks is the Greenwich meridian.
Then, when a plat (US) or plan (UK) is drawn, the distance of a control point from a benchmark is given, as well as the location whose north is being used. If the area is less than 400 km2, the earth can be treated as flat, but the Cartesian coordinates used in surveying do not match the meridians and parallels except at one meridian.
The standard instrument for surveying is the theodolite.
Modern surveying also makes use of laser theodolites and satellite positioning systems.
Types of survey include:
- Boundary survey: The actual positions of existing marks on land (typically iron rods or concrete monuments in the ground, but also tacks in trees, pipes, and manholes) are measured, and a map is drawn from the data.
- Subdivision plat: Given a survey of a parcel of land, lines are drawn inside it, indicating where roads and lots are. Plats are usually discussed back and forth between the developer and the surveyor until they are agreed on, at which point pins are driven into the ground to mark the lot corners and curve ends and the plat is recorded in the county cadaster (USA) or land registry (UK).
- Draw lot: One lot from a plat is drawn, with any easements and setbacks that may be on it.
- Plot plan: A proposal for a house or other building and driveway or parking lot are added to a draw lot.
- Foundation: The position of the house is measured before it is finished being built.
- Physical: The finished house and driveway are measured, and all markers on the boundary are indicated. This is recorded when the lot is sold.