Coordinate systems: comprehensive guide to projected systems in GIS and surveying

Setting up the correct coordinate system is essential when starting a surveying project. One of the most common reasons for faulty data or delays in the field is mismatched coordinate systems—between your rover, base, or NTRIP service.

To avoid costly mistakes, it’s important to understand the difference between geographic and projected coordinate systems and how they work together to ensure a smooth experience with geospatial data.

Understanding geographic coordinate systems

Every time you open the Uber app to request a ride, a chain of geospatial processes kicks in. Your GPS sends your current position to Uber’s servers, while your driver’s location is transmitted in real time. All of this relies on a geographic coordinate system that defines where you are on the Earth’s surface.

Then, software can run calculations across a digital road network database. ‘Where are you?’ ‘Where are you going?’ ‘Where are the available drivers?’ All these data points reference a location within a geographic coordinate system (GCS). These calculations are done quickly and could not be completed without accurate location information.

A geographic coordinate system is a collection of mathematical models that allow us to say where something is in relation to something else on the Earth. If you open the compass app on your phone, you will see a latitude and longitude reading (represented here as degrees, minutes, and seconds). This is your location on the Earth. Without a standardized GCS, these numbers would be meaningless.

The Earth is a large, three-dimensional surface. And while we like to think of it as a sphere, the Earth is more irregular than that, with the poles squished and the equator bulging. The correct shape of the Earth is an oblate spheroid. So, to create functional geographic coordinate systems, we need mathematicians who specialize in the measurement of the Earth, and they are called geodesists. It is these professionals who make the models used in a GCS. 

What parts make up a geographic coordinate system?

There are three components of the GCS: ellipsoid, geoid, and datum. Let’s start with the two that represent approximate shapes of the non-spherical Earth. The ellipsoid is a smooth, squished ball (best for horizontal measurements). The geoid is a bumpy shape, including gravity and topography (best for vertical measurements).

Each geographic coordinate system includes two datums: a horizontal datum and a vertical datum. The horizontal datum uses the ellipsoid for horizontal measurements, while the vertical datum typically uses the geoid for vertical measurements—though in some cases, ellipsoidal height is used instead. The datum defines how the ellipsoid aligns with the geoid. This collection of models within the GCS forms the foundation for the projected coordinate system (PCS), which we’ll explore next.

What is a projection system in GIS and surveying?

To represent the Earth’s 3D surface on a 2D surface, we need a map projection. This projection can represent the entire Earth or a much smaller part of the Earth. At all zoom levels, a projection is necessary. That’s why almost all countries have their own projected coordinate systems (PCS). In the United States alone, there are over 3,000 PCS. 

To imagine the challenge of representing a 3D shape on a 2D surface (printed or digital, flat map), try to remove an orange peel and lay it flat without ripping it. Impossible.

One of the most straightforward projections, the planar projection, works like this (see below): the Eastern Hemisphere is projected onto a 2D surface by placing a light source inside a translucent globe. The grey land mass and black graticules are now shown flattened, and we can view the entire side of the globe.

While there are an infinite number of projections, there are three general categories: planar, conic, and cylindrical. Each type is suitable for different applications. For example, the cylindrical type is used for large-area maps, mainly when depicting areas near the equator, and is generally not recommended for small-area maps.

Remember the orange? It is impossible to remove the peel and lay it out flat without ripping it. Think of these rips as distortions in the final map. Each projection has different distortions, and thus, compromises are necessary for each map projection.

• Conformal projections preserve true shapes and angles of small areas but distort their size. Aeronautical charts and topographic maps are examples.
• Equal-area projections distort shape and direction but display the actual relative sizes of all areas. A population distribution map is an example.
• Equidistant map projections preserve distances, although only from one or two points to any other point on the map or in specific directions. A world map is an example.

Most map projections are mixed compromises, as the mapmaker must decide which distortions are acceptable for their map’s purpose. They attempt to minimize the distortions that most negatively impact their map.

How does the geographic coordinate system relate to the projected coordinate system?

The GCS collects ellipsoid, geoid, and datum information, indicating where something is in 3D space. The projected coordinate system contains the GCS information plus more details like the mathematical model of the projection.

When do you need coordinate transformations?

As you work with geospatial data, you may encounter a situation where different datasets don’t align correctly. That’s where coordinate transformations come in.

Transformations are used in two common scenarios.

1. First, when your data comes from different projected coordinate systems—which is a frequent case, given the variety of PCS in use. A transformation helps convert all your datasets into a single, unified system, so everything lines up as it should.

2. Second, transformations are needed when older datasets were collected using outdated technology. In this case, you may need to adjust the original coordinates. Transformation software uses correction values from a transformation grid and interpolates them to match the location in the new coordinate system.

Curious about how all these pieces fit together—ellipsoids, geoids, datums, projections, transformation grids, and custom coordinate systems? Check out our paper Introduction to Coordinate Systems and get the insights you need to work with spatial data confidently and accurately.

How to use coordinate systems for your survey projects

In the field, you can run into a few scenarios with coordinate systems. We’ll cover the three most common scenarios and how to deal with them using the Emlid Flow app.

1. The coordinate system you need is known and is contained in the Emlid Flow library.
2. You don’t know what coordinate system to use.
3. You have parameters for a coordinate system not defined in the Emlid Flow library, enabling you to create a custom coordinate system.

For this example, we’ll use the Emlid Flow mobile app, and while the interface differs from the Emlid Flow 360 desktop version, the principles and functionality will remain consistent. 

What if you have a known projected coordinate system?

This is the most straightforward scenario. Let’s say you’re collecting data in Esmeralda County, Nevada, and you already have your coordinate system info: NAD83/Nevada West for horizontal and NAVD88 (GEOID12B) for vertical.

In Emlid Flow, there’s a built-in library of ready-to-use, region-specific coordinate systems. You can search by name, country, or code. To set it up:

Tap “New Project” and select Coordinate system. Search for NAD83/Nevada West and select it from the list.

Then, choose the vertical datum—NAVD88 (GEOID12B) height in this case. The app will automatically suggest all available vertical datums that match your chosen horizontal coordinate system.

That’s it! You’re now ready to start collecting data using your preferred coordinate system.

What if you don’t know much about coordinate systems?

You should first ask the client if they have a preferred projected coordinate system. If they don’t have one or if they don’t know how to find it, you should ask them for a previous dataset that they have on hand.

If your client can’t provide you with any sample data and an established coordinate system they work with, you have to do some research. But don’t worry. There are many free and useful resources to help you.

The first place you want to go is the EPSG Registry. It contains definitions of around 6,000 coordinate reference systems and coordinate transformations, which may be global, regional, national, or local in application. Here’s what you can do:

1. In our case, since you are in Kentucky, start by searching in the EPSG Registry for the “[name]=Kentucky.” This search returned 28 results. All the results’ name fields contain either north, south, or single zone.
   
2. For example, if you are near Bowling Green, Kentucky, you may decide to choose the South zone. Now you can narrow your search, filtering only for “south.” But there are still options left.

On the right side, there is a field called “Revision Date,” listing when the coordinate system was revised. In our case, the most recent revision date is in 2018. You choose “NAD83 / Kentucky South (ftUS) + NAVD88 height (ftUS)” or EPSG “8742”

Now, you take that EPSG code and search for it within the Emlid Flow library, and you’re ready to start collecting data.

What if you need a custom projected coordinate system?

Sometimes, your surveying project might take place in a region where standard coordinate systems aren’t a perfect fit. This can happen if local authorities or specific industries use customized datums, projections, or height models—or if your data needs to align with legacy systems or unique client requirements.

In such cases, you’ll need to set up a custom projected coordinate system.

Emlid Flow makes this process straightforward, even if the system you need isn’t listed in the Emlid Flow library. To configure it, you’ll need the following parameters:

• The ellipsoid your datum is based on
• Projection type and its parameters
• Transformation type and its parameters
• Geoid model (if you’re working with orthometric heights)

Use only the parameters relevant to your setup. For example, you can skip the transformation if your projection shares the same datum as your base. If you choose to work with ellipsoidal height, selecting a geoid model is unnecessary.

Ready to set up your custom coordinate system? Follow our step-by-step guide to get started.

Survey with your coordinate systems in Emlid Flow 

Understanding the concept of geographic and projected coordinate systems is crucial to getting accurate survey results. Now that you know how this concept works, you can easily prepare your survey projects using the Emlid Flow or Emlid Flow 360 apps. To learn more about how the app features can help you with your surveying tasks, look closely at Emlid Flow.