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Frequently Asked Questions

Find below some of the more frequently asked questions about the U.S. National Grid (USNG) and the nationally standardized Emergency Location Marker (ELM).

Why can’t the dispatchers determine the caller’s location from the caller’s cell phone?

They can - BUT - the current generation cellphone location in most instances is derived from the GPS chip in that phone. Anything which degrades the phone’s ability to “see” the GPS satellites will degrade the phone’s location determination – inaccuracies which can be considerable. Examples:

  • GPS tracking in buildings is basically nonexistent – this is a huge problem for 9-1-1 services as more and more individuals are using a cellphone as their only phone
  • Forest canopy – same issue as a cellphone in a building; specifically important in the recreational trail environment 
  • Urban and natural canyons – a limited view of satellites by being in an area with a restricted horizon will again result in an inaccurate location report
  • Bad antenna – damage to the cellphone or wrapping it a protective case can impact location determination  
  • Old technology – e.g. 1st generation flip phones; yes, they are still out there and they have no GPS chip
  • Privacy concerns - for privacy reasons, lots of folks disable location tracking.  And, although there are supposed to be 9-1-1 override capabilities, call center issues may limit this capability.

Furthermore, impacting the location accuracy calculation is the receiving system’s capabilities. Is it a 9-1-1, E9-1-1, or NG9-1-1 system? And, how well trained is the staff operating the call center?

Nationwide adoption of NG911 has been slow and spotty, and many rural areas where recreational trails are located remain decades away from implementing. In addition, significant problems remain even in urban areas.  Here are some examples:

Finally, Computer Aided Dispatch (CAD) systems default to latitude/longitude for cellphone location determination, which introduces an additional problem.  There are three versions of latitude/longitude: DD.MM.SS, DD.MM.mm, and DD.dddd. Confusion over which version of latitude/longitude was being used to describe a location needing a response has sent rescuers to the wrong location with deadly results.

What is the downside to creating a local numbering system for trails (e.g. 1a, 2b , 3a)?

Locally unique marking systems have NO VALUE to responders unless those locations are READILY AVAILABLE in dispatch and response systems. Which means your organization will need to constantly update and maintain your unique GIS information in police, fire, ems and ambulance service data bases. One example of how a local system can cause rescue problems occurred in 2010 in a 2,700 acre regional park in Minnesota: Mom, Son and Even Rescuer Lose Their Way at Lebanon Hills Regional Park, July 20, 2010.

Additionally, local numbering systems:

  • Don’t work with GPS like the ELM system,
  • Don’t facilitate a seamless recreational trail user experience – think “stop sign” value of the standardized ELM on trails in different locations – instantly recognizable,
  • Don’t integrate on intrastate and interstate systems crossing multiple jurisdictions; Appalachian Trail as an example,
  • Don’t employ the federal standard for all land-based search and rescue as endorsed by FEMA and the National Search and Rescue Committee – U.S. National Grid (USNG),
  • Don’t easily accommodate adding or deleting location markers. Since each ELM is associated with a unique location on the face of the earth, marker locations can be added or deleted without impacting the rest of the system,
  • Don’t exist as part of a larger, free national standard ecosystem of maps and apps,
  • Don't efficiently empower trail maintenance teams to report specific locations needing work. 


What happens if a person is off the trail – what good is the ELM then?

Unlike a localized trail marking system, there is a way to find an individual who is lost when off a trail provided they still have a working cellphone.

In addition to almost every GPS unit being able to select USNG (listed as MGRS on some units), there are all kinds of apps which display USNG – most notable USNGapp.org. It is a free, browser-based, operating system agnostic, view of your USNG location which looks like an ELM. And, it works without an internet connection after the first load.


What about What3Words and similar approaches (Mapcode, Google Plus Codes, Natural Area Coding System, Munich Orientation Convention, etc.)? Wouldn’t they work better?

The merits of the underlying U.S. National Grid and ELM system over these alternate approaches are substantial:

  • In each commercial instance, you are BUYING into another entity owning your “where” capability, which places you on a developmental dead-end limited by what the provider will deliver,
  • USNG is the free federally endorsed standard for SAR and response which invites follow on innovative development,
  • Proprietary approaches don’t work with GPS, USNG/ELMs does,
  • Proprietary approaches are NOT part of a bigger eco-system of maps and apps as is the case with ELMs – these other proprietary products are their own self-standing “thing,"
  • USNG/ELM coordinates mean something intuitive – Cartesian, distance based – which is why this approach has been used by U.S. and NATO armed forces worldwide since the late 1940’s,
  • USNG/ELMs provide spatial context/awareness and shows relations to objects,
  • Scalable – USNG works to describe locations ranging from submeter to multi-state, this is not possible with these other systems, and
  • USNG/ELMs work with, or without apps, computers or electricity.


Is there royalty or other hidden costs for using this system?


The underlying U.S. National Grid is a non-proprietary cartographic and response standard of the U.S. federal government. Similarly, SharedGeo is a federally recognized nonprofit which retains copyright on the ELM design to ensure it remains in the public domain. If desired/requested, SharedGeo will issue to any entity with trail ownership/operating authority a letter stating SharedGeo releases in perpetuity the authorization to use the design in the entities’ area of responsibility. SharedGeo’s only reason for placing copyright on the ELM design is to ensure it can serve as concept guardian and prevent a commercial lockout/exploitation of trail organizations at some point in the future.

Virtually all supporting documentation SharedGeo has developed for the ELM system is available for free under Creative Commons licensing. Additionally, SharedGeo has developed several USNG/ELM apps which are available to the public for free.