The Earth is not flat and why that affects the accuracy of your coordinates in IrisBG

The combination of decisions you make about collecting coordinates will determine the coordinates' accuracy. You will need to answer the following questions:

• In what datum is your IrisBG database set?
• What are you going to use as a base map?
• How are you collecting the coordinates? 

In what datum is your IrisBG database set?

A geodetic datum is an abstract coordinate system with a reference surface (such as sea level) that provides known locations to begin surveys and create maps, definition from the National Oceanic and Atmospheric Administration. IrisBG supports the following datum: WGS84, ED50, ETRS89, NAD83, OSGB36 and GDA2020. Learn what a datum is and how to find what datum your IrisBG database is using, in this article. Note: your datum cannot be changed once records have been entered into IrisBG.

What are you using for a basemap?

Basemaps are made with a map projection. A projection is the method used to portray a part of the Earth on a flat surface, this can be a paper map or a computer screen. The Earth, we know, is not flat but Google Maps/Bing/OpenStreetMap, and ESRI 2D maps are flat, commonly used, and freely available.  They represent the globe in a 2 dimensional way and using the WGS 84 / Pseudo-Mercator projection. On a large scale map, (closely zoomed in to the street level), you likely will not notice distortions. However, like removing the peel of an orange and trying to flatten the peel, surface of an orange is distorted when asked to become flat. This distortion affects area, shape, distances, and angles.

The best example of this is shown when contrasting how Greenland changes in size on the globe to how it appears on the commonly used WGS 84 / Pseudo-Mercator 2D projection displayed below.

Greenland seen on a Globe above and below in an ESRI map displayed in WGS 84 / Pseudo-Mercator projection

The 2D projection makes Greenland look larger than the South American continent. In reality, Greenland overlayed on South America looks like this:

Thank you https://www.thetruesize.com/ for this helpful tool.

Ok. But I don’t live in Greenland and it looks like the middle of the globe around the equator is not as distorted so I won’t have any issues. Read on…

What device are you using to capture the coordinates?

1. Are you visually placing the coordinates on a map using a computer and the basemap imagery provided by a service? It is important to know that this service may be updated as new imagery becomes available. Google Maps/Bing/OpenStreetMap, and ESRI 2D are in the projection WGS 84 / Pseudo-Mercator or EPSG: 3857 which is used globally and gives errors of 0.7 percent in scale. These services are updated with better imagery at higher resolutions the basemap may shift 1-2 meters or 3 to 6.6 feet. See this scholarly article from the International Archives of the Photogrammetry to learn more.
2. Are you visually placing the coordinates on a map with a basemap you have curated and hosted? This basemap won't shift unless you change the it. It is a good practice to have buildings or survey markers in the basemap to assist in realignment later, if needed.
3. Are you using the GPS in your phone or mobile device? These results vary widely depending on your environmental context. Under an open sky, GPS enabled devices are typically accurate to 4.9 meters or 16 feet. The accuracy worsens near trees, bridges, and buildings. See results from a study conducted by the Institute of Navigation.
4. Are you using a mobile device paired with a Global Navigation Satellite System (GNSS) receiver for better accuracy? Read this article to learn more about GNSS receivers.

• • If you are using a GNSS receiver that uses satellite data to triangulate your location and apply corrections to your coordinates, you may be able to achieve meter and submeter accuracy in an open sky context.
  • Are you wanting the accuracy to be within centimeters or inches and corrected in real time? A real time kinetic network (RTK) is what you need to invest in including: equipment that can process this data and an internet data service that provides real-time corrections.

Just thinking about setting up an IrisBG Database and would like to choose the right datum and map projection? See this article: How do I choose a Map Coordinate System for GIS?