GPS

The GPS tab is where you define the Localization methods, geoid file, and GPS scale factor.

• Base Translation by Rover Readings: When enabled, a local base receiver's position can be "back-calculated" through a rover measurement of a known point. This is commonly used on jobs that span multiple days where the subsequent base location is not known or has not been saved.
• Multi Point Method: When using multiple points to determine the localization, select the desired method:
  • Plane Similarity: Uses a best-fit least-squares transformation and performs a translation, rotation and scale.
  • Rigid Body (No Scale): Uses a best-fit least-squares transformation and performs a translation and rotation, without scale.
  • Helmert (7 param): Requires the user to enter the seven parameters on the By Helmert tab.
• One Point Azimuth: When one-point is used for the localization, the grid coordinate is translated to the local coordinate and the azimuth can be either:
  • State Plane Grid North: The direction parallel to the vertical lines (meridians) of a projected coordinate system commonly used within small areas (like cities or counties) where the distortion is minimal and manageable.
  • Geodetic North: The direction pointing toward the geographic North Pole commonly used in navigation, astronomy, and large-scale mapping efforts where the Earth's curvature is considered.
• Geoid File: This option will incorporate the geoid undulation in determining the orthometric elevation of the measurement.
  More Information

  The National Geodetic Survey definition of the geoid model is the equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level. Orthometric elevations (H) are typically used in survey calculations. In order to convert ellipsoid heights (h) as measured by GPS into orthometric elevations, you must provide for a correction between the GPS-measured ellipsoid (reference ellipsoid) and a constant level gravitational surface, the geoid. This correction is the geoid separation distance (N).

  

  SurvPC will install a few common geoids for North America. Other geoids can be used:

  

  

• Geoid Interpolation Method: Select the preferred method to determine the geoid value based on the geoid grid cell:
  • Quadratic: This method estimates values between known data points using a quadratic function and assumes that the relationship between the data points follows a quadratic curve.
  • Bilinear: Provides a compromise between simplicity and accuracy for interpolating values within a rectangular grid.
  • Bicubic Spline: Provides a balance between accuracy, smoothness, and computational efficiency.
• Use Grid to Ground: When one-point is used for localization (for a translation) or no points are used in the localization file (position is derived from the reference station GPS), the option to Use Grid to Ground is available.
  More Information

  In situations where it is desirable to convert GPS grid measurements to match total station ground measurements, the Grid to Ground scale factor (the reciprocal of the TS - Use Ground to Grid) can be supplied or computed through the exposed Calculate button that launches the dialog box below:

  

  In GPS, the Scale Factor (1/Combined) is applied to the distance between the coordinate and a scale (or reference) point. The ultimate goal will be to supply or calculate a Combined Scale Factor through a desired Method:

  • Read GPS: Determines the combined scale factor based on the location obtained from a GNSS measurement.
  • Manual Entry: Use this when the combined scale factor is already known.
  • Grid Coordinates: Use this if the job contains an existing grid coordinate.
  • Average Points: Use this when two or more points should be averaged (commonly around the perimeter of the site) to derive the combined scale factor.
  • Advanced Mode: Use this option when the Grid Factor and Height Factor are either known or should be independently determined.
  • Scale Point: When not already set through an explicit point from an above method, indicate a reference scale point that will be used for the determination of distances from the scale point to the rover location.

  Note: You can click the "Summation" icon at the top of the dialog and obtain a summary of the scale results.

  

Note: With multi-point localizations, the Use Grid to Ground option becomes grayed out as the scale factor is determined by the localization itself.