What's your latitude?

The utilization of GPS technology allows survey projects to be more readily interfaced into standardized positioning systems, and theoretically, as more and more objects are located by geodetic coordinates, they can be spatially referenced and graphically delineated in their respective positions.  This has allowed the creation of GIS databases that store location information relevant to hundreds of disciplines.  GIS has valuable applications for planning and land use, for resource conservation and development, for navigation and fleet management and for product tracking, etc.  The list is endless.

Because Latitude and Longitude represent spherical coordinates, this geodetic system has the ability to describe the location of points on a globe irrespective of the size of the sphere.  That is, Lat. 39d North, Long. 121W describes the same point on a 22 inch globe, in relation to its Equator and Prime Meridian, as it does on the actual surface of the earth.  Therefore it is necessary to assume a model with a given diameter in order to determine relative distances between given points.  Most boundary surveys and engineering projects are based on plane geometry, assuming a flat surface and assigning Cartesian coordinates (X,Y,&Z) for distance and elevations.  This presents some conversion difficulties when assigning geodetic locations to plane-survey derived points.

Often, land managers, utility companies or other agencies would like to tie their existing GIS or local survey control into an earth-based geometry, or survey data has been collected with GPS receivers and is stored in geodetic format, in which case an adjustment must be made in order to determine inverse relationships between points.  It is important to understand the relationship of absolute (earth-centered) and relative (local positioning) in order to provide meaningful data.

The State Plane coordinate system is useful transforming plane geometry into Lat/Long and vice versa.  There is a direct correlation between these systems and there are a multitude of programs for conducting these transformations.  The most important thing to remember when translating local coordinates to the state plane system is that ground distances inversed from local coordinates need to be scaled appropriately to compensate for the planar to arc, (the combined factor - projection and geoid correction) before the transformation.  The scale factor is close to one, but over distances of several miles it makes a significant difference.  If ground distances are held for the entire project.  I will note the insertion point on the control diagram because it is only at this point that LAT/LONG will be correct.

For determining real-world positions, we either tie benchmarks with known positions into our survey control, or use static GPS data collected and then corrected through the NGS OPUS routine.  This service is provided to allow GPS users access to continuously operating reference stations at known locations.  Simultaneously operating data collectors allow OPUS to compare our information with satellite vectors from distant stations resulting in long baselines and accurate positioning.