Coordinate Systems and the Construction Site

With the growing popularity of GNSS systems for machine control and site positioning in construction, there is a need for a basic understanding of coordinates. During daily operation, operators and grade checkers will not need to work with coordinate systems. However, having someone on site that has a basic understanding of coordinate systems can eliminate downtime and site errors.

 

General Concept

First let’s determine what a coordinate is. The Webster dictionary defines a coordinate as

Any of a set of numbers used in specifying the location of a point on a line, on a surface, or in space.”

In construction, coordinates are normally expressed as Northing, Easting and Elevation and are used in lines, surfaces and features.  On the other hand, GNSS works in Latitude, Longitude and Height in a global system.

Types of Coordinate Systems

There are two basic types of coordinate systems in construction: Cartesian and Spherical.

 

Cartesian

  • A coordinate based on a plane or grid
  • Reported as distance from a origin point, in X,Y
  • Can be expressed globally or locally
  • Typically, either State Plane or a local coordinate

Spherical

  • A global coordinate based on a mathematical ellipsoidal datum
  • In GNSS the WGS84 datum is used
  • Reported as latitude and longitude
  • Not very useful on a construction site

 

Making GNSS Coordinates Useful

One of the first tasks in preparing a site for GNSS operation is to convert the GNSS Spherical values to a useable Cartesian value.  To do this, we need a set of transformation parameters containing the relationship between the two values. These parameters are calculated in two ways.

 

Site Calibration

A Site Calibration is supplied by the site engineers and calculated by surveyors. This is the preferred method. If a site calibration is not available, one can easily be created in the Trimble SPS Controller.

How it Works

During the measurement of the control points the controller calculates a horizontal and vertical adjustment parameter. The horizontal and vertical adjustments are computed separately. Control points can be horizontal and vertical, horizontal only or vertical only. These parameters are:

  • Horizontal
    • Rotation
    • Translation
    • Scale
  • Vertical
    • Block shift
    • Tilted plane
    • Geoid Separation

As control point observations are added, the controller refines all the observations for a best fit solution.  After three observations have been completed the controller will start to display horizontal residuals

Steps in performing a Site Calibration

  1. Load a Comma Separate Values (CSV) file containing the local control point name Northing, Easting, Elevation.
  2. Take a measurement at each control point.
    1. Minimum of 3 points, 5 or more points are preferred.
  3. When finished measuring the control points:
    1. Review the residuals and if within tolerance, accept the calibration.
  4. Export the calibration for use on the machines.

 

Published Coordinate System

Published coordinate systems are based on a global or local definition. One part of the definition is the datum used. A datum can be simply defined as a starting point or origin of a coordinate system. This starting point is usually in WGS84 latitude and longitude. To use a published coordinate system, we must be on the same datum. This can be done in two ways, setup the base station over a known point and enter in the latitude, longitude and height, or measure a known point with the rover as a datum shift. In most cases this would be a ‘US State Plane’ system. The parameters to use a State Plane system are already in the data controller.

Using a US State Plane Coordinate

There are few items to be aware of when using a published State Plane system.

  • A datum shift must be measured prior to using the coordinates.
  • It’s preferred that the base be setup at a control point with known Geodetic coordinates in the State Plane datum.
  • The coordinates are based on a grid that is at mean sea level (MSL).
  • When connected to a GNSS receiver, Trimble data collectors display distances in ground units.
  • If the controller is connected to a Total Station (optical Instrument), the grid to ground scale factor must be used.

 

Elevations

I did not cover elevations in this segment as it would make the document too big to follow.  So, I will cover the vertical component of a coordinate in a later article.

Conclusion

No matter what type of coordinate system your site uses, the Trimble SPS controller makes easy work of the transformation. You just need to measure a few control points and let the controller do the rest.  Once the transformation is complete for a site, you will not need to do it again. Just transfer the site calibration to all machines and controllers. Don’t forget if you run in to problems, SITECH Southwest has a knowledgeable staff to help with any issues you may have.

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