January 2020
If you’re working with drawings created by a surveyor, they will be based off ground coordinates. To use these drawings alongside Nearmap imagery, you need to convert them to GIS (or grid) data.
Photo by Scott Blake on Unsplash
To many of our customers, their primary work flow is to bring all the data for a project into a platform such as AutoCAD or ArcGIS. In most of the cases it’s as simple as attaching a raster or linking to a WMS connection, but sometimes there are extra steps needed when the data they are working with was captured with a ground projection.
Ground coordinate system
A surveyor will collect data points and bring them together into what is referred to as ground coordinates. When you are working on a drawing that is based off this data, you are working on a ground coordinate system. Ground coordinates are measurements that were taken on the actual surface of the earth.
GIS (grid) system
GIS data, on the other hand, operates based on the spatial data’s coordinate system, or “grid” system. It is a mathematical model using an ellipsoid surface to represent the earth’s shape. In order to correct for the difference between these two and bring our imagery in line with the ground measurements, we need to apply a scale factor of both direction and distance.
Figure 1: Model of an ellipsoid surface with a ground projection. info@cad-1.com|www.cad-1.com| 2013 CAD-1.Inc
Direction correction
When a survey is being performed, several variations of north can be used: True north, magnetic north, grid north, or a locally defined north. GIS data will use the north of the coordinate system, so the direction will need to be rotated to convert one into the other. This angle of difference is the direction offset that will need to be added or subtracted to align the data.
Distance correction
Distances measured on the ground are relative to the local ground elevation whereas distance in a coordinate system are measured relative to that coordinate systems ellipsoid (or simply, sea level).
The survey plan should contain the correction information which might be called the basis of bearing or convergence angle for the direction adjustment and the scale factor for the distance correction.
How to apply this correction in ArcGIS and Autodesk?
Figure 2: Ground to grid tool in ArcGIS Pro
Inside of ArcGIS Pro this conversion couldn’t be easier. On the edit ribbon there is a Ground To Grid button that allows you to quickly input the scale factor and apply the correction
Inside of Autodesk the process is a little different but still easy once you know the steps. You will need to know what coordinate system you would like to use as well as the combined scale factor. The combined scale factor is going to be the result of the grid scale multiplied by the orthometric height scale. This number should be included in the drawing provided by the surveyor. Once you have the coordinate system and combined scale factor figured out, you need to create a new coordinate system and multiply the false easting and the scale reduction by the combined scale factor.
For example:
False easting calculation: (1640416.66666667) * (1.0003672940) = 1641019.18186583666789098
Scale reduction calculation: (0.9999) * (1.0003672940) = 1.0002672572706
The methods used to do the grid to ground conversions are sure to vary depending on the platform you are using, but the basic concepts will remain the same. At Nearmap we are always striving to bring reality to your workstation and working with multiple coordinate paradigms is part of that effort. Regardless of whether you work inside of grid coordinates or ground coordinates, you can be confident that you can incorporate high resolution Nearmap imagery into your workflow.
Daniel Finley
Technical Support Engineer South Jordan, Utah
Daniel is a geophysicist with experience in the GIS and satellite worlds.
He likes all things sport and you will often find him tucking into an afternoon chocolate bar.
