The vast majority of the order polygons we are sent and then the data we deliver here at Apollo Mapping is in a projected coordinate system such as Universal Transverse Mercator (UTM). And while projected coordinates systems are typically preferred to a geographic system, you often need to know their zone number to load them quickly in various software applications; you also need to know the zone number if you wish to reproject a spatial file in an alternate projection into UTM. With this in mind, for the October Geospatial Tip of the Month (GTM) I offer four ways to determine the UTM zone number of a spatial file.
A Bit on the Universal Transverse Mercator Coordinate System
Before we jump into the various ways you can determine a UTM zone, let’s very briefly cover the topic of spatial projections. In order to display spatial information in its correct location on the planet, this data must be assigned a projection which is comprised of a coordinate system and datum. A projected coordinate system is a mathematical function that lets us display data from a curved globe on a flat computer monitor or piece of paper; and then the datum is the generalized shape of the planet used in this mathematical transformation. Many spatial data users choose UTM (the coordinate system) WGS84 (the datum) as their projection of choose as it is generally accepted as the most accurate way to represent spatial data. For a more through handling of projections, coordinate systems and datums, you can check out my two-part series here (Part 1; Part 2) or these two web resources (Resource 1; Resource 2).
Option 1 – The Mathematic Approach
If you know the left (or west) most longitude of the dataset in question, then one approach to determining the correct zone number is using good old mathematics. Here is a re-hatch of the technique described in my two-part series above:
- Take your longitude coordinate in decimal degrees and add 180. Most often, people will choose a coordinate in the north-west corner of their data, and assign it this zone number even if the data straddles two zones.
- Then divide by 6.
- Finally round-up to the next highest whole number.
So for example, the UTM zone number for 39° W would be found through these steps:
- -39 + 180 = 141
- 141 / 6 = 23.5
- 5 rounds up to 24
So 39° W is in UTM zone number 24. If the data is above the Equator, then it would be in zone 24 North; and if below the Equator, it would be in zone 24 South.
Option 2 – Check the PRJ File
If the data you have is in shapefile format, it is already projected to UTM and there is a .PRJ file with it, you can simply open the PRJ file in a text editor like Notepad or Wordpad to find the zone number. The zone number would look something like this in a text editor:
So we are in zone 28 North in this example.
Option 3 – Check the Zone Number in ArcGIS
If you have access to a copy of ArcGIS (or a similar GIS software application) and a shapefile of UTM zones, you can simply load the UTM zone grid file and the spatial dataset in the same session. Then click on the Identify button to see what zone number your spatial dataset falls inside of. If your dataset crosses over various zones, we suggest using the zone number where the upper-left most corner falls. This is probably the best way to determine your zone number if the dataset is a raster file.
Option 4 – Check the Zone Number in Google Earth
If your dataset is a KMZ/KML file or you can convert it to a KMZ/KML (for instance by using Image Hunter for the conversion), then you can load it in Google Earth to find the zone number. Be sure your units are set to Universal Transverse Mercator in Google Earth – if they are, you will find the zone number at the bottom center of your screen when you zoom in on the KMZ/KML.
In the video that accompanies this article, I will walk you through the steps to complete Options 2, 3 and 4 above. Well, that was a short, but sweet, GTM!
Do you have an idea for a future GTM? If so, let me know by email at firstname.lastname@example.org.
Brock Adam McCarty