A coordinate reference system (CRS) is crucial for accurately defining and comparing locations on Earth by providing X, Y, and Z coordinates. There are two main types: geographic CRSs, which use latitude and longitude on a spherical surface (e.g., WGS84, NAD 1983, GDA2020), and projected CRSs, which flatten this 3D data onto a 2D plane using mathematical calculations (e.g., UTM, US State Plane). CRSs can be published, meaning they are standardized and publicly available, or local, which are arbitrary grids specific to a single worksite. Knowing and consistently applying the correct CRS is essential for data comparison and accurate measurements in various applications.
Four key components build a Coordinate Reference System
- A coordinate system,
- Horizontal and vertical units,
- A datum, and
- A projection.
Knowing your coordinate reference system is essential when:
- setting up a new site
- using a known point to process your AeroPoints data
- Setting up a local grid with a JXL or GC3 file for your site
- uploading any preprocessed data
- downloading data from Propeller
- comparing survey data to design surfaces
If the CRS of one dataset does not match the CRS of another dataset or design, you will not be able to compare them. So, if you upload preprocessed data or a design surface with a specific CRS to Propeller, you must set up the site to match it. If you don’t know what CRS your worksite uses, you should ask your surveyor.
You can always change a site's CRS to match a design file or other data. If you need to convert a point from one CRS to another, you can use Propeller's free online coordinate converter.
Different types of coordinate reference systems
Geographic
A geographic coordinate reference system uses latitude and longitude to define the location of points on the surface of a sphere or spheroid. The definition of a geographic coordinate system includes a datum, prime meridian, and angular unit.
Geographic coordinate systems are sometimes called "datums," though they are technically separate concepts. You can learn more about datums here.
Common examples
- WGS84 (Global)
- NAD 1983 (the US)
- GDA94 and the recently published GDA2020 (Australia)
Projected
Projected coordinate reference systems use a planar Cartesian (flat, two-dimensional surface). They refer to the mathematical calculations performed to flatten the 3D data onto a 2D plane and are based on a geographic coordinate system.
Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across two dimensions. Therefore, all projected systems are subject to distortion in shape, direction, size, area, angle, or some combination of these factors.
For example, look at the infamous Mercator projection to understand the distortion issues.
Common examples
- US State Plane Coordinate System
- Map Grid of Australia
- Universal Transverse Mercator (UTM) system
Published vs. local
Published coordinate systems are standardized systems available and usable by the public (such as those in the EPSG database). There are published geographic and projected coordinate systems.
Local systems are always projected coordinate systems. They are arbitrary grids that are specific to a single worksite.
If you do not already have a CRS for your worksite and your drone surveys don’t need to be compared to design files, it is better to use a published coordinate system when setting up a new site. You can work with your customer success representative to ensure your sites are set up correctly.
I still can't do it!
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