Coordinate Reference System

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 sherical 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 rectangluar coordinates and displays the equivalent geodetic coordinates based on the CRS.

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

Ground vs Grid Distance in the Traverse View

The Traverse View allows you to use a distance factor to relate grid distances to the distances it displays. The distance factor is displayed on the status bar when you highlight a distance and is printed out with the traverse data.  TPC uses the following formula in the Traverse View. You assign a combined scale factor (grid factor X elevation factor) to a CRS. In the Traverse View, you can tell TPC to use this combined scale factor as the distance factor. Each traverse has it's own distance factor that is stored with the traverse, so it is very easy to create one traverse on the grid for computing area and another traverse at your project elevation to check measured ground distances. You'll find lots of uses for the distance factor.

When recalling points in the Traverse View and displaying inversed data,

• distance displayed in the Traverse View = inversed grid distance / distance factor (for that traverse)

When entering measured distances in the Traverse View and reducing field data (measured distances) to grid coordinates,

• grid distance used to compute a point on the rectangular coordinate system = measured ground distance X distance factor

Specifying a distance factor for a traverse takes care of relating the distances displayed in the Traverse View to the grid. If you want to work strictly on the grid, leave the distance factor set to 1.000000 (the default). Remember, all rectangular coordinates in TPC are grid coordinates. So the distance factor always starts or ends with grid coordinates.

Ground vs Grid Distance in a Drawing

When you label lines in a drawing, you can apply a distance factor just like you do in the Traverse View. The traverse settings that are used to draw each traverse include a distance factor.

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 Components

Each CRS has the following components.

Name

The name defines a record type in the TpcZone.dat file that holds all the CRS definitions. This is a text file that you can edit to add or modify CRS components. TPC ships with the following reference systems:

Local, UTM, NAD27, NAD83 (1986) (Conus), OCRS (Oregon Coordinate Reference System) and WISCRS (Wisconsin Coordinate Reference System).

Zone

A CRS can have any number of zones. Zones define a projection and datum. TPC includes the common zones for North America and world zones for UTM. Zones for the OCRS and WISCRS reference systems are also included.

Ellipsoids

Ellipsoids are methematical representations of the Earth. TPC provides several common ellipsoides like Clark 1866, GRS 1980, WGS 66, WGS 72 and WGS 84 (not to be confused with the WGS 84 datum). These are defined in the TpcEllipses.dat file in the program folder. You would not normally modify this file since you can easily edit ellipses directly from TPC.

You can easily modify the existing ellipsoids or add your own directly from TPC using the Ellipse dialog. The ellipses you create or edit are stored in the UserEllipses.dat file in your program data folder.

Horinzontal Datum

A horizontal datum (datum) relate the geodetic coordinates in a CRS to the rectangular coordinates. Hundreds of datums exist around the world, each relating a geodetic coordinate to a specific rectangular coordinate. Published geodetic reference points in your area will include rectangular coordinates and the name of the datum those coordinates are based on. The datum you choose to use in your survey may be dictated by state law (statues).

Traverse includes the following horizontal datums in its TpcZone.dat file (in the program folder). UTM, NAD27 and NAD83 (Conus) or more specifically NAD83 (1986) (Conus). You can modify certain datum parameters like the false origin and reference latitudes and longitudes in the TpcZone.dat file if needed.

Horizontal Datum Shifts

TPC does not automatically shift horizontal datums. If for instance, you transform your survey from NAD83 (1986) to NAD27, you will need to manually translate the survey to a published position on the NAD27 datum.

Vertical Datum

TPC does not currently use any mathematical goid models to compute elevations. Think of these as traditional elevations or more specifically as orthometric heights (ellipsoid height - geoid height).

Elevations are assumed to be based on the National Geodetic Vertical Datum of 1929 (NGVD 1929) which is generally considered to be mean sea level.

Conversions between geodetic and rectangular coordinates are done indpendent of a points elevation.

Projections

A map projection or just projection projects the ellipsoid onto a flat surface. TPC supports the following projections

• Lambert conformal conic (with 1 or 2 standard parallels)
• Transverse Mercator
• Universal Transverse Mercator
• Oblique Mercator

The projection is defined for each zone in a CRS. So within a CRS, each zone can use the projection which best fits that zone.