What is a surveyor? Why survey land?

Under British law- land cannot be sold without a 'title'. Title means that there is usually a piece of paper or a legal document to show that a  person or persons own the land.
Under Maori tikanga, land ownership was not under individual title - but belonged to a hapu or iwi. In other words, the whole group owned it together.
Surveyors measured land and drew up plans that they would pass on to cartographers. Cartographers are people who make the maps.
Surveyors looked for arable land- or land suitable for farming. They laid out plans for towns and cities and searched for new areas for people to live in. They looked for the best places to put roads and looked for places for fresh water and harbours for ships to anchor in. 
The plan was to measure and map the land for the Crown (Government) or a Company
 (The New Zealand Company). In Taranaki, the aim was to map the land so that blocks of land could be sold off to settlers to develop farms.