The US National Grid System
For those not familiar with USNG, here's a brief summary, with links to more in-depth information at the end of the article.
The US National Grid System (USNG) provides a precise address for any location in the country, down to as small an area as one meter by one meter, or about 3.3 feet by 3.3 feet, somewhat smaller than the average automobile parking spot.
This can be very handy if you have a GPS device that reads at that level of precision and are working your way back to where you buried the gold.
Here's a typical USNG coordinate, which is the address for the center of the Washington Monument in Washington, DC 20006
USNG: 18SUJ2348306479 (NAD 83)
Starting at the right end, the (NAD 83) is not part of the address but is sometimes included to indicate the data source being used, which in this case indicates they we using the most current data available.
The fifteen character alphanumeric string in the middle is the USNG address and it has three distinct sections. The first five characters indicate a very large section of the world and so it will not likely change while out walking about, searching for a specific coordinate.
The next ten numbers indicate the local area and these are the numbers we'll be working with most often. The first five numbers of that ten number group indicate an east/west orientation, while the second five number group indicates the north/south orientation.
A ten digit numbers offer the highest level of precision, down to the one meter size. But not all USNG addresses use all ten numbers; they can use less.
For example, a neighborhood size area of 1,000 meters by 1,000 meters has only four digits in the address; a soccer field of 100 meters has six digits; a house-size area of 10 meters has eight digits; and a bicycle-size parking spot of 1 meter uses all ten digits.
Incidentally, since these numbers are meters, it becomes possible (with some practice) to calculate how far away another landmark might be if it's USNG address is known.
The topo maps of the Canyon mining claims are provided in both USNG and Longitude/Latitude coordinates, for those who prefer one over the other, but when getting the coordinates from the National Map it seemed that USNG offered a much finer level of precision over Long/Latitude.
In the field, it's desirable to have a GPS device that can use WAAS satellites effectively. Otherwise the accuracy can be off by several meters.
We used a DeLorme PN-20 Earthmate device, which has been able to lock on and hold WAAS very well. And during an accuracy test it was able to locate old government survey markers to within one meter of the marker.
Here are two good articles on USNG and likely more can be found with a quick Internet search.
Introducing USNG by Mike Price
How to read a USNG Spatial Address