# Geolatte-geom

A geometry model for Java that conforms to the Simple Features For SQL specification 1.2.1.

It is intended as a drop-in replacement for the Java Topology Suite (JTS) geometry model. GeoLatte-geom is fully interoperable with JTS but offers the following additional features:

- immutable data structures (Geometries are value objects).
- support for 2D, 3D, 2DM and 3DM geometries
- support for several dialects of WKT/WKB (Postgis, Sql Server, SFS 1.21)
- pluggable, extendable Geometry operations
- CRS-awareness (knowledge of coordinate reference system (projected/geodetic, angular units of metres)
- geodetic operations (length, distance and area calculations)

# Redesign 0.x to 1.0

This version is a complete redesign. The redesign is aimed at:

- increasing the level of type-safety in the API;
- the ability to incorporate geographic (or geodetic) data without duplication of the Geometry class hierarchy;
- making full use of the information on coordinate reference systems that is now available in the CRSRegistry.

## The Coordinate Reference System model (crs package)

The availability of the CRSRegistry, and the explicit modelling of coordinate reference systems, made it obvious that a better way was available for handling the different dimensions of a Geometry's coordinate space. In previous versions this dimensionality was specified with a DimensionalFlag. Having a complete model of the Coordinate Reference System, including it's coordinate space, means we no longer need this. Rather than associating a DimensionalFlag with the Geometry, we only need it's Coordinate Reference System.

Since in the Coordinate Reference System model Projected and Geographic systems are distinguished, we can use a single Geometry class-hierarchy, and let the implementations inspect the associated Coordinate Reference System to determine which operations are valid, or which algorithms are selected (e.g. geodetic length rather than length in 2D plane).

## Base and compound Coordinate Reference Systems

The CRSRegistry only provides access to base systems, which are always 2D. Users can add axes (Vertical or Measure) to a base system, and so create a Compound system. Vertical axes are axes with a Direction of UP (DOWN), measure axes have direction OTHER or UNKNOWN. Although it is possible to add several Measure axes, many measure operations will only take the first such axis into account.

## Positions

A Position is essentially a tuple of coordinates which together with a Coordinate Reference System specify a position in that Coordinate Reference System.

In previous versions, Points played the role of Positions. The concept of a Position, distinguished from a Point, was introduced to have different types of Positions, each corresponding to a type of Coordinate Reference System.

In this new model, a Geometry is conceptually a set of Positions (all associated with the same Coordinate Reference System). The set is determined by one or more sequences of Positions and a type enum value (GeometryType) that determine how the sequence(s) determine the Geometry (e.g. for LineString it is by linear interpolation between the consecutive positions).

## De-emphasizing the Simple Features Specification (SFS)

This library started as an attempt to have a JTS-interoperable library that is SFS-compliant, but has a more modern design and better support for geometries not in the 2-dimensional projected plane. As design progressed, it became no longer obvious what the advantage of SFS compliance are.

With open source and more expressive languages, the advantage of standardisation on the API level are becoming less-and-less obvious. The only advantages to the SFS model (or it's more complicated cousin SQL/MM-Part 3) that I can see are a familiar Geometry model and a precise specification of topological relations. These advantages are offset by the disadvantages of a bias to 2D planar coordinate systems, and a (by current tastes) problematic API design. (Should complex operations really be part of the Geometry interface? What with alternative algorithm implementations?)

Because of these misgivings, we will de-emphasize SFS compliance.