Ground to Grid Problems No More

This time saving tip comes to you from Brett Morlok, a project engineer in our Water Resources practice, to which I’ve added some additional details.


 

Do you want an easy way to manage features that in that are in ground coordinates when everything else in ArcGIS is in grid? If you are new to the idea of ground to grid coordinates, this 2008 paper from Richard J. Sincovec does a great job explaining the ins and outs. However, if you’re mostly interested in how to problem solve when things don’t show up as expected, you’ve come to the right place.

One approach to get everything to line up correctly is to use AutoCAD to scale the features to the grid coordinate system.

Or, you can take the simpler, faster route of simply creating a new coordinate system that is scaled to the county that you are working in. Then, each feature class that you have in ground coordinates can be assigned the new coordinate system to immediately appear where it should. It’s just a few minutes of upfront work that can save a significant amount of time (and headaches) over the course of a project.

Here are some 2-ft LiDAR contours for Richardton, ND shot in ground coordinates. Notice how they are just slightly off from the grid imagery in ArcGIS, especially around the roads.

1

 

In the Catalog window, right click on your feature class and go to the Shapefile Properties. Select the XY Coordinate System tab, right click on the current coordinate system, and select Copy and Modify.

2

 

Next, you will determine what the new meters per unit value should be. Copy the original meters per unit value from the Projected Coordinate System Properties, then multiply by the scale factor for your county.

(original meters per unit value) * (scale factor) = new meters per unit value

To enter it into your Projected Coordinate System Properties, change the Linear Unit Name to <custom>, paste in the new meters per unit value, and save the projection as something unique. Our practice is to keep the coordinate system name the same, and append “_Ground” to the end.

3

 

Now your feature class will show up in the correct spot. In this example, the contours now have a realistic alignment.

4

This methodology is a flexible way to work with a team where some may prefer ground coordinates and others prefer grid coordinates. Any new feature classes in ground can be assigned the newly created coordinate system to appear exactly where they should, in grid.

Leave a Reply

Your email address will not be published. Required fields are marked *