In the second part of this edition of the Geospatial Frequently Asked Question (G-FAQ), I continue the discussion started last month of geographic versus Universal Transverse Mercator (UTM) coordinate systems. In last month’s G-FAQ, the focus was on the history of each coordinate system as well as the basic question, why UTM zone number is important. For October, I focus on the advantages and limitations of each system to give our readers some advice as to when each is most appropriate.
With this in mind, I address these core questions in this two-part G-FAQ series:
Is this UTM zone number important? How are UTM different than geographic (latitude and longitude) coordinates? When should I use each coordinate system?
The Geographic Coordinate System
What we learned last month is that a geographic coordinate system is technically not a map projection as the coordinates have not been modified to be displayed ‘properly’ on a flat 2D surface. This single fact is at the root of the advantages (and limitations) of a geographic coordinate system.
- A geographic coordinate system is good for locating an exact position on the globe as each coordinate is referenced to the surface of our planet. This ‘exact position’ is crucial for shipping given how remote the open oceans are.
- Geographic systems are truly global as they include the poles – many projected coordinates systems are only valid over a portion of the earth’s surface.
- Latitude and longitudes can be determined with astronomical observations and an accurate time keeper. Projected coordinate systems do not offer this as they are completely arbitrary – another huge advantage when navigating the open oceans before the digital era.
- Since a geographic system is truly global, you can measure very long distances which cannot be done in projected systems. That said, measurements taken in a geographic system on a paper map would not be accurate as you are measuring straight lines on what should be a 3D surface. When measuring these distances in ArcGIS, they should be more accurate as geospatial programs can take this into account. I say should as it is very difficult to impossible to verify measurements of long distances.
- With geographic coordinate systems, it is possible to define a great circle which is the shortest distance one can travel on a globe (i.e. the Earth) between two points. A great circle is created by drawing a plane that intersects the two points on the globe and the center of the planet. Where it touches the surface of the globe is the great circle defining this shortest distance. Now in reality, the surface of our planet is obviously not flat, so the true ‘great circle’ path would have local variations as it travels over mountains and down valleys.
- As you move north and south from the Equator, the diameter of the planet narrows so that at the poles, it is significantly less distance around the globe than at its center. As such, the distance between one longitude degrees shrinks from about 111 kilometers (km) at the Equator to 0 km at the Poles where all longitude lines converge! This means that trying to measure distance based on subtractions of latitude and longitudes value alone is not possible (it takes more complex geometry).
- Similarly, since geographic coordinates are not planar, linear measurements on a map in this system are by definition inaccurate.
So the long and the short is that geographic coordinates systems are ideal for plotting exact positions on a globe; however they are challenging and misleading to use for straight line measurements that many of us try to (understandably) make with maps.
The Universal Transverse Mercator Coordinate System
Now let’s take a look at the advantages and limitations of a UTM coordinate system. As UTM is a projected coordinate system, its advantages and limitations are nearly the exact opposite as for a geographic system.
- UTM is a cylindrical projection created by encircling our relatively spherical globe with a cylinder that touches only at the Equator. Each UTM zone is then created by projecting a narrow slice of the globe onto the 2D surface of the cylinder. When this is done, shapes that fall inside a single zone have an accurate representation and even larger shapes have minimal distortion. Within 10 degrees of the Equator, there is only about a 1% error in shape. As you move to the poles, the distortion does increase.
- As shapes are well maintained in UTM zones, the distortion in area calculations are also minimal. Similarly, local angle measurements are true as are distances.
- When the world is divided into zones, scale can remain relatively consistent across them. As mentioned with geographic coordinate systems, as you move towards the pole, the distance between 1 degree longitude changes dramatically, you do not see this change in UTM zones.
- The limitation of all projected coordinate systems are large shapes that cover parts of multiple zones. When a shape crosses into more than one zone, it will have increased distortion. Shapes that span more than two zones are not meant to be projected in a UTM system.
- Without a UTM zone number and North or South, a set of Northings and Eastings could fall in any one of 120 locations on the planet. This is a definite limitation if coordinates become separated from zone number info!
- The UTM coordinate system only covers from 80 degrees South latitude to 84 degrees North, an obvious limitation for mapping in polar regions.
- As mentioned above, distortion increases as you move to the poles with noticeable impacts in the northerly and southerly parts of our globe. Scale is still maintained relatively well for small shapes, even close to the poles.
A very simple way to look at using geographic and UTM coordinate systems is this: if you are working on a small area (say less than 500 kilometers across), then UTM is likely the way to go. And if you are working in an area larger than this, then geographic is likely the way to go.
Do you have an idea for a future G-FAQ? If so, let me know by email at [email protected].
Find Out More About This Topic Here
- Brown University – Coordinate Systems & The Role of Projections
- Idaho State University – Introduction to Topographic Maps
- National Park Service – Datums & Coordinate Systems Presentation
- Northern Arizona University – Coordinate Systems Workbook
- University of Wisconsin, Green Bay – The Universal Transverse Mercator System
Brock Adam McCarty