Purpose
This article provides background information on how map projections and coordinate systems work. Understanding these concepts helps ensure that georeferenced plans in CivilPro align correctly with GPS data and field measurements.
Introduction
Before working with georeferenced plans in CivilPro, it helps to understand a few key mapping concepts. Plans that are georeferenced use real-world coordinate systems so that their positions align accurately with GPS data and survey measurements. The following overview explains how map projections and coordinate systems work, with a focus on the Australian context — including MGA zones, GDA datums, continental drift, and the use of Well-Known Text (WKT) definitions.
What is a Map Projection?
A map projection is a mathematical model that translates the Earth’s curved, three-dimensional surface into a flat, two-dimensional plane — such as a digital map or printed plan.
Because the Earth is roughly spherical, no projection can represent all distances, angles, and areas perfectly. Each projection makes trade-offs depending on its purpose — for example, some preserve distances accurately over small areas, while others are better for representing large regions.
In practical terms, when you see a georeferenced plan (for example, a drawing that aligns with GPS positions), the plan’s coordinates are based on a specific projection system that defines how latitude and longitude are converted into X and Y positions on the map.
Coordinate Reference Systems (CRS)
A Coordinate Reference System (CRS) defines not only the projection, but also the datum — a mathematical model of the Earth used as the reference for latitude, longitude, and elevation.
Every point in a CRS has coordinates that are meaningful only in relation to that CRS. Two maps showing the same location can differ slightly if they use different datums or projections, even though both are “correct” in their own context.
MGA and GDA in Australia
In Australia, spatial data is commonly defined using the Map Grid of Australia (MGA), which is based on the Universal Transverse Mercator (UTM) projection system. The MGA divides the country into zones that are six degrees of longitude wide, numbered from west to east.
| Region | Typical MGA Zones |
| Western Australia | 50–51 |
| Northern Territory | 52–53 |
| Queensland | 55–56 |
| New South Wales | 55–56 |
| Victoria | 55–56 |
| Tasmania | 55–56 |
The MGA coordinates are tied to a geodetic datum — specifically the Geocentric Datum of Australia (GDA).
There are currently two main versions:
- GDA94 – based on the Earth’s position in 1994.
- GDA2020 – updated to reflect continental movement since then.
Australia moves northeast by approximately 7 centimetres per year due to tectonic motion. Over time, this drift causes discrepancies between GPS readings (which reference a global datum) and fixed national coordinate systems. GDA2020 realigns Australia’s map grid with the global coordinate system as it was in 2020, reducing errors between GPS-based positions and local mapping data.
GPS and Map Coordinates
GPS devices use a global reference system known as WGS84 (World Geodetic System 1984). This is very similar to GDA94, but because of continental drift, the difference between GPS coordinates (WGS84) and MGA94 coordinates can be up to 1.5 metres in some locations.
GDA2020 was introduced to close that gap.
When a plan or drawing is georeferenced, it typically specifies its coordinate system in a standard text format so that software can interpret it correctly. This ensures that when GPS data, drone imagery, and engineering plans are combined, all spatial data aligns accurately on the same coordinate grid.
Well-Known Text (WKT)
A Well-Known Text (WKT) string is a standard way of describing a coordinate reference system in a human-readable format.
For example, an MGA Zone 56 (GDA2020) projection might be described as:
PROJCS["GDA2020 / MGA zone 56",
GEOGCS["GDA2020",
DATUM["Geocentric_Datum_of_Australia_2020",
SPHEROID["GRS 1980",6378137,298.257222101]],
PRIMEM["Greenwich",0],
UNIT["degree",0.0174532925199433]],
PROJECTION["Transverse_Mercator"],
PARAMETER["latitude_of_origin",0],
PARAMETER["central_meridian",153],
PARAMETER["scale_factor",0.9996],
PARAMETER["false_easting",500000],
PARAMETER["false_northing",10000000],
UNIT["metre",1]]
Mapping software, GIS tools, and survey applications use this definition to correctly interpret and transform coordinates between systems.
Why It Matters
Understanding projections and coordinate systems ensures that when spatial data is plotted — whether it’s survey data, drone imagery, or engineering drawings — everything lines up correctly.
Using the correct CRS avoids problems such as:
- Misalignment between plans and GPS data
- Features appearing in the wrong location when overlaid
- Errors in distance and area measurements
When creating or importing georeferenced plans, it’s important to know which MGA zone, GDA version, and projection are used, and to ensure consistency across all data sources.
Summary Table
| Concept | Description | Example |
| Projection | Mathematical model that flattens the Earth’s surface for mapping | Transverse Mercator |
| Datum | Reference model of the Earth’s shape and position | GDA94, GDA2020 |
| MGA Zone | 6° longitude-wide section across Australia using UTM projection | MGA Zone 56 |
| WKT | Text format describing a projection and datum | PROJCS["GDA2020 / MGA zone 56", …] |
| GPS Reference | Global coordinate system used by satellites | WGS84 |
| Continental Drift | Australia moves ~7 cm/year NE, causing coordinate offsets | Corrected by GDA2020 |
How This Relates to CivilPro
In CivilPro, plans can be georeferenced so that features placed on them align with real-world locations. When you upload or configure a plan, CivilPro uses the plan’s coordinate reference system — defined by its projection, datum, and zone — to interpret positions correctly. Alternatively, you can create this georeferencing from CivilPro yourself using our Plan Registration tool.
This allows data captured in the field using GPS devices or other spatial tools to match accurately with the locations shown on the plan. Ensuring that your plan uses the correct MGA zone and GDA version helps maintain precise spatial alignment across your project.