Your Pro Citizen Newsletter 32 - Map Datum and Why it Matters, Weekly Discount Code

Map Datum and Why it Matters

By Jack Morris

A map is simply a graphic representation of a portion of the earth's surface drawn to scale, as seen from above. The challenge the mapmaker (cartographer) has is to take a 3D sphere with irregular shapes and make a flat two-dimensional image with references (grids) so it is useful for navigation. The grid system we use has to start somewhere, there must be a standardized baseline or start point. On a topographic map the reference start point is referred to as map datum. A datum is a description of the shape of the earth along with an anchor point for the coordinate system. This datum has a specific established data set that is annotated by year on your map’s marginal data. NAD27, NAD83 and WGS84 are the common map datums in the US.

Yawn. That is the cartographer’s problem, why do I care mister map nerd?  

Datum differences matter. Consider two comparison photos of two maps with different datums; the two pictures below are the same piece of terrain, the same grid square. Take a look at the grid lines compared to the terrain. The historical map on the left uses North American Datum 1927 (NAD27) while the newer one on the right uses North American Datum 1983 (NAD83). Notice how the grid intersection southeast of Hill 2545 has shifted a couple hundred meters south due to using the newer datum.

If you enter the wrong datum in your GPS it can result in significant navigation errors. It will also be a problem if you have hiking partners or teammates using maps with different datum. If you choose to use historical topo maps for your navigation, be sure to account for the grid differences if you are transferring location data to electronics or newer maps. Quick side note, you can read the new CM-5 Land Nav manual for a detailed discussion on historical series topo maps and their advantages and disadvantages.

Depending on where you are in the US the difference between NAD27 and NAD83 can vary from 10 meters all the way up to 200 meters. The difference between NAD83 and WGS84 is less significant at around 1 meter - but you should still account for it.

 Just use WGS 84 big sarge. This was the “default” or go to when the GPS units were becoming available in the Army (late 90s). The ignorance of the system and the importance of map datum caused some issues. The default “just put in WGS84” was a bad habit many military GPS users got into, but as you saw with the example above doing so can cause significant navigational errors. Check your map datum and use the one that aligns with your map version.

 New!!! Land Navigation Manual

CM-5 Land Nav manual preorders are live today (link below)! This manual is laser focused on the foundational land nav skills you need in the real world. The Pro Citizen land nav manual covers everything you need and nothing you don’t. To keep this work relevant and useful for the reader Jack was very deliberate about choosing the subjects and the depth which each is addressed for the book. Over 200 color pages with descriptions and clear explanations of practical land navigation skills. (preorders will ship the last week of August)

Some of the CM-5 topics Include:

Nav Tools and Detailed Descriptions

Combining compass and map skills

Planning navigation routes and terrain analysis

Terrain Association

Mounted Land Nav introduction

Land Navigation Manual CM-5 – The Professional Citizen Project (tpcproject.com)

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^{Use of any DoD photos does not indicate endorsement}