Coordinate Reference System

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Coordinate Reference Systems

Each survey has a Coordinate Reference System (CRS) which determines the relationship between geographic coordinates (latitude and longitude) and rectangular coordinates (X and Y). The CRS uses an ellipsoid (mathematical representation of the earth), a horizontal datum (to reference geodetic to rectangular coordinates), a vertical datum (currently only NGVD 1929) and a map projection (to convert the earth's spherical surface to a flat surface).

TPC will sometimes refer the rectangular coordinate system of a CRS as the grid. All rectangular coordinates are assumed to lie on the grid. So an inverse between two points is a grid inverse. TPC does most of it's computations with rectangular coordinates and displays the equivalent geodetic coordinates based on the CRS.

You can visit http://www.spatialreference.org/ to find Coordinate Reference Systems in any number of formats, including ESRI references, including PRJ files which TPC reads.

Local Site (Ground) Rectangular Coordinate System

The default CRS for any survey is a rectangular coordinate system with no geodetic reference. If you are checking a lot closure or generating a topo map for a building lot and don't need to tie the survey into a geodetic control point or State Plane Coordinate System the local grid is all you need.

Typical CRS Uses

Your usage of a CRS will vary depending on the survey. Here are some examples of what you might want to do:

•Start a survey by entering one or more reference monuments by their geodetic coordinates. TPC computes their equivalent grid coordinates and off you go.

•Reduce ground shots (field data) to grid coordinates using a combined scale factor for the survey.

•Compute two new grid coordinates based on two geodetic reference monuments then rotate and translate your survey to match those points.

•Import geodetic coordinates from your GPS receiver or software.

•Transform rectangular coordinates based on another CRS into the CRS your survey is using. Now you can use those coordinates in your survey.

Grid vs Ground

The CRS you end up using will affect how you work with grid vs ground coordinates. TPC has tools to help you work efficiently with both grid and ground.

Cartesian Grid

It is not uncommon to work with a survey as if it is at ground, even if it uses projected coordinates, like those in a State Plane or UTM CRS. These surveys can be referred to as Site or Ground surveys that use Cartesian grid coordinates with no distance or angle corrections.

Low Distortion Projection

If your CRS is a low distortion projection, like those used by a county or other small geographical area, you may choose to consider working with Cartesian grid coordinates with no distance or angle corrections as appropriate.

State Plane or UTM

For Coordinate Reference Systems like State Plane and UTM, grid coordinates and distance are not the same as ground coordinates and distances. Again, TPC has all the tools to help you work efficiently with these differences.

Grid (Theta) Angle in a Drawing

•In any drawing, choose Tools | Drawing Settings and left click the Miscellaneous tab.

•Enter the appropriate Theta angle.

TPC will add the Theta angle to the direction labels for all survey lines in the drawing. If you were to choose a representative Theta value from your survey and enter it here, you would in effect go from displaying grid North to geodetic (true) North.

CRS Definition Files

TPC stores CRS data in two types of files, CRS and PRJ.

CRS files are unique to TPC. They store all the zones for a known coordinate reference systems like NAD 83, or UTM. See CRS Files.

PRJ files are ESRI Projection files associated with shape files. If they contain a PROJCS[] section, TPC can create a CRS you can use for your survey. See PRJ Files.

These files are stored in the program data folder, which can be accessed with the Browse or Open button in the Coordinate Reference System dialog. Moving or copying prj files into this folder makes them available to TPC.

Site Calibration

TPC's Site Calibration automatically computes calibrated coordinates for geodetic points. This in essence, allows you to use two coordinates systems in TPC at the same time.

For example, you can set up your survey to use a State Plane or UTM CRS, but work entirely in the local site (ground) coordinates of a construction project at starting coordinate point 10000/10000. Just tie some project points with your GPS / GNSS equipment, use them to create a calibration and TPC does the rest. Now all the points you tie with your GPS / GNSS equipment will have local site coordinates in TPC. And when you go to stakeout points, TPC just reverses the process, creating the appropriate geodetic position for each local site point you export.

See Calibrations and Coordinate Reference Systems.

Geoids

TPC supports a number of geoids that it uses to compute elevations from ellipse heights and visa versa. The current geoid model, including None, is displayed in the first field of the Geoid group box.

By default, TPC sets the geoid to None, meaning TPC treats ellipse heights as elevation and visa versa. If however, you select a geoid, TPC automatically starts using that geoid and it's septation from the ellipsoid to compute the geoid undulation, also called geoid height.

•Geodetic points will then compute a corresponding elevation based on the geoid and their ellipsoid height.

•Coordinate points with then compute a corresponding ellipsoid height based on their elevation.

For more information see Geoids.

Survey (Project) Location

A CRS uses your survey or project location for most of its computations. TPC may require additional information about this location to use the CRS that you have selected.