HP 206 Researching Historic Structures and Sites
Geographical coordinate system project standards
It would be great if we only had one standard geographical coordinate system, but there are many ways that geographical data are being recorded and used, especially now when standards, tools and resources are changing so rapidly. Here is some background information.
The UTM NAD 83 datum a commonly used standard for recording the geographic locations. This is the same standard that is used by the UVM Landscape Change Program. Please note that for our barn census project project, we will be using 911 addresses instead of UTMs, however for such historic preservation work as documenting locations for National Register nominations, UTM coordinates are used.
Here is some background:
Datum: To comply best with current GIS resources, including the standards of the Vermont Center for Geographic Information, use NAD83 This is the datum standard that most GIS mapping in the US is using or migrating to. Just remember that older maps may have used the NAD27 datum.
(For more information on the VCGI standards see http://www.vcgi.org/techres/standards/partii_section_b.pdf )
The NAD83 (North American Datum of 1983) and WGS84 (World Geodetic System of 1984) datum-- which is a common default setting on many GPS units -- are only very slightly different, with variations generally only showing up beyond the levels of accuracy of our GPS units. These differences are based on slightly different mathematical models of the ellipsoidal shape of the earth.
UTM: To comply with current GIS resources, record the UTM coordinates using the NAD83 datum. The UTM system, which was developed for military purposes, projects the shape of earth's surface onto a rectangular grid.
Latitude and Longitude: To comply with the Landscape Change Program standards, let's also record latitude and longitude in degrees, minutes and seconds (also using the NAD83 datum). This is the default for many GPS units and GIS mapping resource. Latitude and longitude can also be expressed in decimal degrees as we have also seen, but for compatibility, let's record our viewpoints for each image in degrees, minutes and seconds.
Here is an example of the latitude and longitude coordinates for Wheeler House: Lat 44° 28' 36" Lon -73° 12' 05" (The longitude is sometimes shown as a negative number since Vermont is west on the 0° Prime Meridian that passes through Greenwich, England.)
Just remember that latitude is the north-south angle. The Vermont - Quebec border runs close to the 45° latitude which is half way between the equator and the North Pole.
The US Geological Survey (USGS) has a web site called the National Map that provides access to GIS maps and photos. It now provides USGS Digital Orthophoto Quarter Quadrangles with 1-meter ground resolution of most of the United States and its territories. It is free and does not require special software. It does provides latitude and longitude readings, but these may not be quite as accurate as the readings we can get by using a GPS unit.
National Map at http://nationalmap.gov/
Google Maps at http://www.google.com/maps
Google Earth at http://earth.google.com/