What Are Ground Control Points?
Ground Control Points, or GCPs, are marked points on the ground that have a known geographic location. In aerial surveying, a drone can be used to autonomously collect photos of the survey area. If used, GCPs must be visible in these aerial photos. The photos are then processed in the cloud using drone mapping software. A common solution for collecting and processing drone data, used by many mapping professionals, is DroneDeploy. Services like DroneDeploy create 2D orthomosaics, 3D models, digital elevation models (DEMs), and 3D point clouds from drone images. For aerial survey applications GCPs, are typically required as they can enhance the positioning and accuracy of the mapping outputs. To do this correctly, the GCPs and desired mapping outputs must use the same ESPG code.
What is an EPSG code?
The European Petroleum Survey Group (ESPG) collected and standardized a large list of Spatial Reference Systems. An EPSG code is simply shorthand for the full definition of a specific spatial reference system.
Why does everything use EPSG codes? Because the consortium of companies that formed the ESPG had the most to gain (ie. finding lots of delicious oil) from creating really simple and accurate guide to spatial reference systems so they took the time to compile the best database of reference systems that has since been adopted by other companies, governments, ESRI, and nearly all GIS professionals.
Spatial Reference Systems for GCPs
To properly communicate a known geographic location, such as the position of a ground control point, we need pick a spatial reference system. A spatial reference system defines the 5 following parameters.
2) REFERENCE DATUM
The reference datum is the reference from which all positions are measured. In 2D grids this is the point (0,0), also called the origin. In spherical or ellipsoidal grids this is the prime latitude and longitude lines (eg. the Equator and Prime Meridian).
4) UNITS OF MEASUREMENT
The units of measurement specify how geographic locations are measured and recorded with respect to the reference datum. Common units for aerial surveying are:
- Degrees (Lat/Long)
- US Survey Foot
The Tricky Business of Measuring GCP Elevation on Earth
Measuring GCP elevation from the center of the Earth is impractical so instead we measure elevation from the more useful reference, sea level.
As sea level is constantly changing, this extremely complicated! However, geodetic scientist have created a model that can define sea level to within 1 meter anywhere on earth. The accomplish this by building a geopotential mode of the Earth consisting of spherical harmonic coefficients. This model is called a geoid and it is tells us where the theoretic sea level is everywhere on earth.
Check out the video below for an excellent overview of how sea level is measured.
These models are very difficult to create and as such there are very few that are widely used for GIS. The current and standard is the Earth Gravitational Model 1996 (EGM96).
Map Projections: Displaying a Sphere on a Flat Screen
SHOWING THE EARTH ON YOUR COMPUTER SCREEN
As the earth is a 3 dimensional ellipsoid, we need to project it onto a flat surface so you can view it on your computer screen.
There are many ways to do this and they all warp the size and shape of the Earth in some way.
The standard for web mapping applications such as Google Maps, Bing Maps, OpenStreetMap, ESRI, Mapbox, and others is Web Mercator (EPSG: 3857).
Bringing it all together with WGS84
To bring everything we learned about spatial reference systems together, let’s go over the World Geodetic System, WGS84 (EPSG: 4326) the most common EPSG Code and the standard for all satellite navigation systems including GPS (USA), GLONASS (Russia), Galileo (Europe), and BeiDou-2 (China). We will compare this with the North American Datum of 1983, NAD83 (EPSG: 4269) the most common EPSG Code for construction and survey in North America.
WGS84: IERS REFERENCE MERIDIAN AND THE EQUATOR
NAD83: NAD83 REFERENCE DATUM
WGS84: EARTH GRAVITATIONAL MODEL 1996 (EGM96)
NAD83: EARTH GRAVITATIONAL MODEL 1996 (EGM96)
WGS84: DEGREES (LAT/LONG) AND METERS (ELEVATION)
NAD83: DEGREES (NORTHING/EASTING) AND US SURVEY FEET (ELEVATION
WGS84: NO PROJECTION
NAD83: NO PROJECTION
And that's it! If you've made it this far you're sure to be a master of GCP spatial reference systems.
Click the button below to move to the next section on using GCPs for drone mapping.