A coordinate reference system (CRS) defines the location of a point on a plane or a sphere.
It provides the X, Y, and Z locations for all points in space and allows you to determine where you are on Earth consistently.
Without a complete definition of that system (all dimensions), you can’t accurately figure out where you are in relation to other locations or measure between them.
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 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 are uploading preprocessed data or a design surface with a specific CRS to Propeller, you must set up the site to match it. You should ask your surveyor if you don’t know what CRS your worksite uses.
You can always change a CRS for a site if you need to make it match a design file or other data in the site. You can use Propeller's free online coordinate converter if you need to convert a point from one CRS to another.
Different types of coordinate reference systems
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.
- WGS84 (Global)
- NAD 1983 (the US)
- GDA94 and the recently published GDA2020 (Australia)
Projected coordinate reference systems use a planar Cartesian system (a flat, two-dimensional surface). The projected systems 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.
Look at the infamous Mercator projection to understand the distortion issues.
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!
We wrote these articles to equip you with everything you need to get the job done on your own, but we understand that sometimes this isn't sufficient.
If you're stuck, the Propeller data success team may be able to help. You can contact our support team by clicking the support tab on the top-right pane of your user portal.