Wednesday, June 12, 2013

Day 8- Putting Skill on the Map

Day 8: Putting Skill on the Map

Slowly but surely, catching up with the daily blog, for Tuesday’s lecture Dr. Neiman presented a lecture on datums, map projections, and grids. Most people when they think about archaeology would immediately think about finding artifacts in exotic foreign lands, and well, yes that is true, but another major part of archaeology is being able to know a lot about geology. Since we, as archaeologists, spend our time in the dirt, we know that it can hold a great deal of facts or secrets within its many layers, if you just look close enough to find them. Dirt of course, is just one small element on the bigger picture of Earth, so first we need to have a little background in geodesy.

Geodesy is the science of measuring the shapes of the earth. Traditionally in the past, archaeologists used to use grid systems (local grid systems) to set base lines in order to dig, which meant randomly assigning a zero point (0,0- thinking back to basic geometry). Today however, thanks to the wonderful advances in technology we have the satellite and GPS (Global Positioning Satellite), the once used arbitrary grids formed by archaeologists have been swapped for Real Earth Grid Systems.

Isaac Newton was the first to state the earth was not exactly circular, but rather that it formed an ellipsoid. In the nineteenth and twentieth centuries, different ellipsoid models were developed for different parts of the world in order to make map estimate plan tangent to the curved surface of the earth. A level plane is perpendicular to the local direction of gravity, which varies by very small degrees in different regions, and how specifically gravity varies is described by the geoid.  The geoid is the level at which gravity is sea level. Again, with the technology of satellites, measurements of modern ellipsoids are able to fit global “bumps” in the geoid, making it applicable for the entire world to use. This is done by taking the gravametric center of the earth with satellite technology, and the global geoid used today is called World Geodetic System-84 (WGS84, from 1984).

When discussing datums is it important to keep in mind that latitude and longitude are major and minor axes defined on an ellipsoid fit locally or globally to the geoid (WGS84). In turn, the poles and the equator can be defined as well. Poles are the axis of the revolution of the ellipsoid and the equator is the mid-way between the poles spanning the widest part of the ellipsoid. Together, these define latitude as a natural measurement. Longitude is defined as arbitrary, it has no natural starting place on Earth’s surface- the zero degree point is the Greenwich Meridian in England. The place where latitudes and longitudes are actually located on the ground is termed the “Geodetic Datum.” The Geodetic Datum is a network of controlled points on the ground whose locations are given in terms of estimated latitude and longitude. In order to create a geodetic datum you have to specify an ellipsoidal path on the model of the Earth, and create a network of carefully surveyed locations estimating the latitude and longitude. With WGS84, newer datums are geocentric, meaning it is useful anywhere, whereas the older datums were based on locally ellipsoids, isolating their use. In North America, the North American Datum of 1983 is our geocentric reference ellipsoid locally for our continent, and the World Geodetic Survey of 1984 (created by Doppler satellite) serves as the geocentric reference ellipsoid for the entire world.  

These datums are most commonly used for map projections, which is the process of transferring points from the surface of the ellipsoid to a plane. All map projections created some distortion- some are better than others. Which projection is best, depends on the shape of the area you are trying to map (map surface does not equal ellipsoid surface). There are three projections surfaces: cylindrical, conic, and Planar (Azimuthal) and three projection orientations: equatorial, transverse, and oblique. As well as two different kinds of possible contact: tangent, being in contact at a single point along a line, and secant, cutting, or intersecting the surface.

Here at Monticello, there are two main projections we need to know: the Lambert Conformal Conic and the Transverse Mercator Projection. 

The Lambert conformal conic, means that a cone wrapped around the earth, with the projection surface touching the ellipsoid surface along two standard parallels, and this projection has the least amount of distortion runs along the east and west parallels. The Transverse Mercator Projection means that a cylindrical projection surface touches the ellipsoid surface along one central north and south meridian, and the least amount of distortion is along the north/south parallel. Within the United States, the U.S. State Plane Coordination System is used as an underlying projection, utilizing the Lambert Conformal Conic for states with large east/west areas, and the Transverse Mercator for the states with large north/south areas. On an even larger scale (again), the Universal Transverse Mercator Coordination System (UTM) is the underlying projection for the world.

Now, how does all these datums and map projections relate to archaeology? I’m about to show you. The bottom line is, you need to know your map projections and datums so that you can know what grid your site is on, in order to fully, and accurately record your dig. In the case of Monticello, it is also useful to know that the local grid system (VSP- Virginia State Plane) will be easy for your archaeological research design, than using the global grid (GPS). Officially, Monticello is under the N. Am. Datum of ’83, the Lambert Conformal Conic Projection, and the Virginia State Plane System. So the next time you look at a map- try to test your new geodesy, datum, and map projection skills!

P.S. I will be posting the blog from Monday's fieldtrip to Williamsburg and James Fort by Friday night- with pictures!

1 comment:

  1. Sounds like you are having a great time! Thinking about you!