From the Grid to the Ground–a Conversion Walkthrough

Let’s talk about points, the grid, and the ground. Why not? All the surveyors are – everyone wants to project their grid points onto the ground, or vice versa, and so many users think that they’re doing the right thing when they’re really accomplishing nothing. Let’s take a look at the complete process for making this happen. Follow the link to read on…

I have a point file – very simple, four points, they outline a structure that’s very special to me (can anyone guess what?) Before I located these points, I tied into the local state plane coordinate system – Alabama SPCS, East Zone, NAD83, USft. Here are the points:

1,764808.783,761979.103,640,SW_COR
2,765063.382,762137.783,640,NW_COR
3,764978.753,762273.569,640,NE_COR
4,764724.154,762114.889,640,SE_COR

Now – when most users insert these points, their first step will be to go to the drawing settings dialog and select a coordinate zone, just as I’ve done below:

 

 

The really savvy users will then go to the transformation tab and select a transformation. Maybe they’ll apply a sea level scale factor, but mostly they’ll specify a grid scale factor. There are multiple choices, each user has one that they use for a specific reason. Once all that is selected, most users will now import their points using the good old standby PNEZD (Comma Delimited) format. They’ll even select “Do Coordinate Transformation if Possible.” And nothing will happen.

Why?

Because Civil 3D doesn’t know it has anything to transform. Again, why? Because you haven’t told it. You need to set up a custom point file format and tell it exactly what you have – grid coordinates in a specific coordinate system! How will it know what to transform TO if it has no clue what it’s transforming FROM?

Let’s take a look at the point file format dialog – you may be familiar with it, you may have even played around with it a bit – but there are a few things you might not know about it.

 

 

Here, we’ll start at the top and work our way down. First, we’ll give it a name (AL83-EF,) then a file extension (.txt,) then a coordinate zone (AL83-EF,) and the delimiter (,)

Now comes the juicy part – we have to pick what each column is. To do this, pick the <unused> header of each column to bring up the data types. You’ll pick each data type just like I’m showing:

 

 

What’s that? Grid Northing and Grid Easting? That’s right – we can tell the point file format that we’re using grid coordinates so it’ll have something to transform! Once you’re done, you can load up a point file just to make sure it parses correctly and you’re ready to save it.

Now we move on to importing the points – you’ll specify your point file, pick the point file format, and then select Do coordinate transformation if possible (it will be.)

 

 

Now when you bring the points in, you will see that your ground northing and easting is listed along with the grid northing and easting – and as a check, the Latitude and Longitude is displayed as well (checking it in Google Earth puts me right where I want to be!)