With the increasing proliferation of GIS Technology and GPS enabled devices, it is increasingly getting common for everyone to deal with latitude/longitude (lat/lon) on a daily basis. However, a simple, yet important, about lat/lon is often missed by users of GPS devices and also by the developers of mapping application. Most users and developers assume that lat/lon of a point (or landmark such as building) is sufficient to find that landmark on a map (such as google map). To some extent this is true, but not completely. For instance, consider these two geographic coordinates (latitude, longitude) : (1) 47.62054, -122.34947 and (2) 47.62054, -122.348646. If we try to plot these two geographic coordinates, the two points are separated almost by 95 meters (see google map below). The two points are reference for Space Needle building in Seattle, WA but using different datums. Simply speaking, a datum defines shape and size of the Earth and also initial point of reference from which all lat/lon measurements are made. The first coordinate is based on NAD83 datum and the second is based on NAD27. However, since google map doesn’t have the option of specifying datum, both were plotted assuming that both the coordinates are based on same datum. As a result, the two markers (each referring to one of the above geographic coordinates) are separated almost by 95 meteres on the map. However, in reality, two markers should overlap.
Geographic Coordinates for Space Needle, Seattle, WA
1. 47.62054, -122.34947 (based on NAD83 datum)
2. 47.620719, -122.348646 (based on NAD27 datum)
Over the centuries, many researchers have proposed many different datum, each suitable for different model of the Earth. For instance, in 1927, US started following NAD27 for which the reference point was located at Meades Ranch in Kansas. NAD27 is based on elliposidal (Clarke Ellipsoid) model of the Earth. A elliposidal model assume that the shape of Earth is similar to an Ellipse. But with improving technology, researchers had access to better tools and technology, which led to more accurate models of the Earth. Most researchers now follow GRS80 model of the Earth. Based on GRS80 model of the Earth, a new datum, NAD83, was proposed in 1983 for use in US. Research on an accurate model of the Earth and most suitable datum is still ongoing and many changes to NAD84 have been already proposed. But these changes are still not in use.
The crux of all this discussion to emphasize the need for specifying datum when reporting lat/lon. However, if datum information is missing, below are some guidlines on what to assume:
1. By default most GPS uses WGS84 datum. Thus, if you are getting data from GPS equipment and no datum information is available, assume it is WGS84.
2. By default use WGS84 datum if your mapping application can be used by anyone for any geographic region. WGS84 is based on GRS80 model of the Earth, which is most accurate model of the Earth.
3. If you’re mapping application is only for US then use NAD83. All USGS data is reported based on NAD83 datum and is the best for US.
4. If the geographic data is for US, but very old (such as historical data), it is better to assume that it is based on NAD27 datum.