A 340Kb PDF of this article as it appeared in the magazine—complete with images—is available here: Geodesy: USNG: Getting it Right the First Time

USNG: Getting it Right the First Time

Like the proverbial mustard seed that grows large, every once in while a small, supposedly insignificant, detail in a project grows, almost on its own, into a time consuming conundrum. Such was the case for me recently. We were contracted to perform a survey for Texas Department of Transportation (TxDOT) that included setting a few control monuments; something we’ve done hundreds of times before. The conundrum grew out of naming the control points.

The TxDOT district had decided to begin assigning names to new control points by using the United States National Grid (USNG) 10m geoaddressing convention. The USNG is a method of designating a position with one string of numbers and letters that has its origin in the Military Grid Reference System. It is very simple to use and growing in popularity, especially by public agencies and many GIS users. It is based on the UTM projection with its inherent map scale issues and so its advantages are not immediately evident to the land surveying professional.

The USNG joins a header and the last few digits of a point’s UTM x and y coordinates to designate to 1m, 10m or other precisions the location in a single string. I studied the method briefly and created a script in MS Excel to do the work. Being a surveyor I understood that for the point to be located within 10m, I needed to use the value in the ones’ column to round the proper value for the tens’ column; drop the ones’ column and keep the previous four columns for each coordinate. By joining the two new 4 digit numbers I’d have the name for my point in the USNG. All was well … or so I thought.

Our project manager attended a seminar on the subject and returns describing how to truncate the ones’ column. I caught the meaning of the word immediately and hoped it had been used casually and carelessly in the seminar, but he assured me it was used repeatedly. I needed to find the correct method with certainty or face the possibility that some of the points may ambiguously be named two ways! This would frustrate the whole purpose of adopting the convention, so I needed to do some homework.

Darryl Zercher, RPLS with TxDOT referred me to FDGC’s website for their USNG specifications. The site was down for a couple of days and I looked around and found some information at NGS’ website. NGS also had an on-line script and a downloadable program to convert coordinates to USNG. I tried some of my test data felt very pleased with myself because they both gave the same answers as my own script … they rounded. I found an article by Tom Terry, USMC retired, in which he gave an example of rounding USNG values to achieve best accuracy in some situations. He said that the coordinate format allows a full coordinate value to be truncated and abbreviated. I shared all this by e-mail with Mr. Zercher. He almost simultaneously had sent me by e-mail the document I couldn’t get from the FGDC website. It definitely said to truncate. All my best references had resolved nothing!

I did some more research including some instructions on using the USMG. These documents were written at a very elementary, layman’s level. They didn’t mention rounding or truncating but I could tell that it was important for the system to be understood by someone who may be nearly math illiterate. It describes “moving right, then up” to lines that form the grid. It was now obvious to me that the system’s origins called for locating the lower left corner of the quadrant in question, truncating.

I needed to find out why NGS rounded, so I made a phone call to Charles Charllstrom, Director of NGS. He shared my curiosity for the differences between his agency’s and FGDC’s procedures. Saying he enjoyed running down a mystery he would investigate and get back to me. Later he confirmed that the correct method was indeed to truncate and that the agency’s scripts would be corrected and an article like this would be beneficial to get word out to users about the corrections. That conversation was followed by another with Mr. Terry who referenced the USNG standard that describes truncation in Annex B Truncation of USNG Coordinate Values. He joined the now unanimous chorus of truncation.

The FGDC standards may be found by using links on the website: http://www.fgdc.gov/standards/documents/standards/xy_proj/. The documents one should read are: TFIGURES_7.pdf and fgdc_std_011_2001_usng.pdf. All surveyors should at least be familiar with the requirements. Those who may be called to use the system should read them carefully.