What Is The Difference Between Grid and Ground

What is Ground?
While the earth is an ellipsoidal shape in general (at sea level), as you get higher above sea level the “ellipsoid” gets bigger - so a distance on the ground when measured at ground level gets longer the higher above sea level you get. So a Ground Distance is the “measured distance” between two points on the actual earths surface.

What is Grid?
To produce Maps we use Coordinate Systems which are made up of Projections and Datums. The Datums define the ellipsoidal definitions for the Coordinate System and the Projection method is how the Ground Positions are Projected onto a Map / Grid (Cylindrical, Conical, Transverse Mercator etc.). All coordinate systems have an origin, and where you are in relation to that origin will determine the scale factor at that location, based on the Projection and Datum properties of the coordinate system.

Converting Ground to Grid
When you convert a Ground Distance to Grid Distance you typically use a combined scale factor, that combined scale factor is unique to where you are on the earths surface and which coordinate system is being used to compute the Grid. It takes elevation into account (height above sea level / the ellipsoid) and the selected location in the coordinate system into account.

For example in North America, we use the State Plane coordinate system e.g. United States State Plane 1983 and in that we use the NAD 1983 (CONUS, Molodensky) Datum Transformation and we select the Zone e.g. Texas South Central 4204 (where 4204 is the accepted EPSG number for the coordinate system zone definition). The Datum Transformation defines the Ellipsoid Axes and the transformation parameters for the Datum. The Projection defines the name of the projection used as well as the parameters for the projection (Origin, False Northing and Easting of the Origin etc.

When working with Background Images or Street Maps they are always computed and adjusted to fit the Grid, whereas when we do Survey Work or Design Work, they will often be designed to measured locations and are at Ground. If you know the Scale factor of a location you can convert Ground Distances to Grid Distances by Dividing the Ground Distance by the Scale Factor. Same for Coordinates, the Northing at Ground divided by the scale factor will give you the Northing at Grid.

Mismatching Design Data and Map Data
If you have design data loaded and it doesn’t match the TBC Background Map / Image, the likelihood is one of the following


This is an example of a Ground to Grid Shift between the Red Design Alignment and the Background Image Service in TBC.

  1. The Design is at Ground and the Map is at Grid and you will need to shift the Maps to match the Design using Local Site Settings. The Scale Factor can be computed using Local Site Settings, you can then compute the Grid Coordinates (N,E) and then determine the shift and apply the shift as adjustments to the False Northing and Easting of the coordinate system in use. Alternatively you can compute the Shift roughly (good enough to get an overlay to work), by measuring from the Map / Image Location to the Actual Location and differencing the coordinate values of the two points and applying them as adjustments to the False Northing and False Easting of the coordinate system in use using the Local Site Settings command.

  2. The Design is shifted to keep coordinate values smaller. In this case you can apply the known shift as an adjustment to the False Northing and False Easting values in the Local Site Settings Command.

Is The Scale Factor Consistent Everywhere On My Project?
For the purposes of construction on Site Projects (small to medium size site projects) the assumed answer here is that a single scale factor can be used for the entire project. That however does not mean that is 100% true. The reality is that at every point and every elevation there is effectively a different scale factor, however working like that is impractical. So for most jobs you find an Average Elevation to work with (the mean height of the project) and an average location (typically near the center of a project or at least the center of the most critical part of the project), at which the scale factor is determined, and then that same scale factor is used across the entire project and assumed to be constant.

When you get into production you will measure your Known Control for the project with GPS and then the Site Calibration works out the best fit of the data and determines its own scale factor for the project.

When you work on long linear projects, scale factor variations cannot be assumed to be constant, and so either variable scale factors are applied, or you use a specialist projection defined for the project alignment that enables a scale factor of 1 to be used within a distance from the centerline greater than the project will encounter (e.g. a Snake Grid). This makes it easy for construction to operate for the full length of the highway with a single scale factor.

**Do I Need To Be Concerned With This For Site Takeoff?"
Not really. You need to work with the Digital Design provided, and if you want to use the Map / Satellite Background Images in TBC then you can apply the Shifts and for most Site Projects that will give you a good enough fit to put the project in context in TBC or as an output to Google Earth.

This becomes an important issue when you get to construction operations, and in these situations, if you have any doubts, call in a professional surveyor to assist you get your control and site calibration worked out.

3 Likes

As a follow on to the above discussion the following is also something that people ask - what is the difference between a State Plane coordinate and a Modified State Plane coordinate.

The difference is that a State Plane Coordinate is typically a ground coordinate and a modified state plane coordinate is a grid coordinate - i.e. the data has been scaled to account for the Ground to Grid conversion.

You will often see a scale factor written as 1.000108964 or as the reciprocal value 0.999891048 - what is the difference here

The reciprocal value is 1/1.000108964

To go from Ground to Grid you scale using the reciprocal value i.e.,
Ground N x 0.999891048
Ground E x 0.999891048

To go from Grid to Ground you scale using the provided scale factor i.e.,
Ground N x 1.000108964
Ground E x 1.000108964

The scale factor is computed at a specific location (N,E value), so you typically use that location as the origin of the scaling not 0,0.

Lastly we have a Local Coordinate System - the Local System can also be Grid or Ground but instead of having a coordinate N,E range within the standard coordinate range for the State Plane coordinates for that location they will have values that are quite different - for example a State Plane Grid coordinate could be

N=2080387.050
E=6788746.047

and the Local Coordinate could be

N=20000.040
E=20000.110

because the surveyor used a N=20000, E=20000 coordinate as their origin for the survey work carried out prior to design.

1 Like

In the enclosed video, we show how to follow a surveyors instruction on how to Scale, Move and Rotate a model from one coordinate system to another. We use the following TBC commands to do this

Scale Command
Move Command
Rotate Command

You can also use the Rotate, Translate, Scale by Points command if you have two points in the “From System” and 2 points in the “To System” to work with - this is a good way to work as there is a check on the data being translated. It is also helpful if you have some additional point or line data in both systems to act as a second check.

1 Like