MARKS
Marks, Nodes and Coordinates
A Mark record in Landonline represents a physical mark in the real world. A Node record in Landonline is a coordinated point where lines meet. A node is often, but not always, physically occupied by a mark. In some software systems, marks, nodes and coordinates are almost synonymous.
Treating marks and nodes as synonymous works if you are not trying to maintain the history of marks and their positions over time and across multiple surveys.
Landonline needs to differentiate between marks and nodes because:
- A disturbed mark has two nodes - its position before it was disturbed and its position after it was disturbed (see example in the diagram below)
- Some nodes have two marks - eg. if a mark is destroyed and replaced with another mark in the same position (see example in the diagram below)
Landonline needs to differentiate nodes and coordinates because:
- A node can have a current coordinate in several coordinate systems (eg NZGD 2000, NZGD49)
- A node can have several coordinates over time (Landonline remembers historical positions)
Every node needs a set of authoritative coordinates in the official coordinate
system to show where (in the real world) it is. The official coordinate
system is NZGD 2000. A node can only have one authoritative set of coordinates
in NZGD 2000 at a time.
Nodes need observations so that their position can be maintained by network adjustment. Every mark needs a node (otherwise Landonline can't show the position of the mark).
The database needs a mark entry for each node in order to be able to maintain some of the related data, such as names. Therefore Landonline has an entry with a type of Unmarked Mark for cases where a node has no corresponding physical mark in the real world (eg unpegged boundaries, natural boundaries).