Coordinate transformation inside of Civil 3D - working with NTV2 files

THORSTEN SCHEEL: Welcome to everybody. We want to speak a little bit about coordinate transformation inside of Civil 3D. And we were working with NTv2 files, a special point in Civil. My name is Thorsten Scheel from computers, and this is my profile.

I'm application engineer for AutoCAD, over more than 38 years experience. And therefore, I'm one of the most senior application engineers worldwide. Since 33 years, working as a programmer and application engineer for infrastructure and GIS. I also have extended the work in conception and maintenance for Topobase and AutoCAD Map 3D industry models. 27 years I'm experienced in document management on different systems, and for more than 10 years, I'm employee of the Contenlos GmbH in Gehrden in Germany.

So let us start with the theme. At first, when you're looking at transformation of coordinates inside of Civil 3D, you find there's two different ways of transformation [INAUDIBLE]. The first one was the parametric transformation.

Possible are a four, six, seven parameter transformations. And here is has an example of seven parameters. The first [INAUDIBLE] one was moving along an axis for x, y, and z. The next [INAUDIBLE] one was rotating around the axis for x, y, and z. And the seventh parameter was the global scaling.

The coordinate equivalency becomes greater the further you go to the edge of the zone of the projected coordinate system. And there are problems with the transition between two zones. This depends on the projected coordinated systems [INAUDIBLE] definition.

The second way of transformation was affine transformation using NTv2 [INAUDIBLE]. For that, you need two corresponding point grids. They are required.

The point distance must be constant in each direction. So when x, you can have a different one than in y. That's no problem, but inside the direction, it will have to be constant.

For each NTv2 file you will have to define definition area. And the point is valid for this file when it is inside the definition area. You can use at the same time a couple of NTv2 files.

They can be side by side. They can be stackable or overlapping. And it is a good style to [INAUDIBLE] forward back transformation when the point you are searching for is outside of every definition area. The possible coordinate equivalency depends on the distance between the point grids.

Let us now have a look of the technique and why you're using NTv2 for transformation. The National Transformation version 2 was defined in Canada. The format and the method are general and are supported in many GIS programs.

Instead of the date transition through the seventh parameter transformation between projected coordinate systems, this is done with NTv2 by adding shift values to the ellipsoidal coordinates. The exact amounts of coordinates to be transformed are determined by bilinear interpolation within the grid. Rough clue for the accuracy of the NTv2 transformation depends on the homogeneity of the area.

In Germany, we often used the BeTA2007 NTv2 file, which has a grid width of about 11 kilometers. When we use this, we have accuracy in centimeter or decimeter. In some areas, you can achieve a currency in centimeter [INAUDIBLE].

And with a different GSB file with a grid width less than five kilometer, you can reach accuracy in centimeter. And then we use an NTv2 file with a grid width less than 500 meter, we reach millimeter. So you see, depending on the grid width of your file, you can get better results. That's a reason to implement own NTv2 files with a smaller grid width.

Grid files in NTv2 format usually have the ending ASC or GSA. Both are ASCII. Also allowed is the GSB format [INAUDIBLE] the binary storage. Autodesk Civil 3D and AutoCAD MAP 3D can only use the binary NTv2 files. ASCII formats are not mentioned.

In Germany, it's a BeTA2007 NTv2 normally in use for the transformation between the German cordinate system, Grauss-Kruger, and the European ETRS89-UTM. This NTv2 file has integrated with Autodeks Civil 3D and AutoCAD MAP 3D by default. When you use an NTv2 file, the transformation is working in four steps for each point.

The projected source point is transformed to the ellipsoidal, not projected, LatLong coordinate. The second point is to check if the LatLong coordinate is inside the definition area of this NTv2. If not it's using the fallback transformation with the original source coordinate.

The third step is shifting the transformed coordinate using the NTv2 file with target coordinate in LatLong. And the final step is the shifting. ellipsoidal Lat-Long coordinate is transformed to the projected target coordinate to get the final point.

These first steps are produced for every point be transferred. When you use, for example, a line, it's two points. So both points will be transferred if the line was drawn between the two points.

So how can I implement a new additional NTv2 file? When I want to do this, I have a little example at this point. But first, we have to copy the new GSB file to the right place.

For German GSB files, [INAUDIBLE] to the directory ProgramData, Autodesk, eospatial Coordinate Systems, this point 14.08, and the [INAUDIBLE] Germany. The number, the path, may be different. It depends on the version of the Civil 3D you are using.

And you see in the right in the picture where the place is. And in this example, I copied the GSB file country loss inside the directory. So let's have a live look on how to implement it in the system. The command we need is to call the coordinate system library coming with Civil 3D tool.

So there are two possible ways to reach it. First, we can get the normal command line and give it the command. Another way is to change the working area from Civil 3D in planning and analysis, going to map setup. And there in this place, you find the library. And a command for that, you see it in the command line, this map CS library.

The next step is to unselect all filters. And now we select the [INAUDIBLE] transformation. The coordinate system library is a [INAUDIBLE] over all coding systems in the world.

So we have to attach set [INAUDIBLE] second [INAUDIBLE] criteria this point in RDM [INAUDIBLE] the German coordinate system. Now we select normally the second one. Of course, the second one is working with the beta 2007 GSB file.

The first entry you find cannot edit directly. You can view it, but when you would, when you view it, you will see it's a parametric transformation. So we select a second one, go to edit, and get a new formula which we have to select with data, file interpolation. Here you see beta 2007 GSB file.

The next step is important, and we have to do it first before you try to add the new file. You have to define the file format. And in this case, we've selected Canadian NTv2. Important is that you at this point select the extension, the possible extensions for adding files.

The next step is pushing the [INAUDIBLE] button. Now we have to go to the place where we copy the new GSB file-- this case, Germany. And we find [INAUDIBLE] GSB.

We opened it, and it is added. Now you have, this case, two GSB files. And [INAUDIBLE] is the ranking. When searching for a GSB file and proving the area, it started at the top. It goes down, the first GSB file, where the right area was hit and all others are ignored.

Therefore, we have first to go to our [INAUDIBLE] GSB file with the final grid. And with the error button, we move it on top. OK, then we have a little look. At this point, it was a fallback.

You can say, OK, we didn't need any fallback transformation. When we do this and we find the point, it's outside the transmission area. Then this point was ignored or also the object in the point [INAUDIBLE].

Let me define here what back transformation. Then we have the chance to get about select [INAUDIBLE] the Select button. You see we are going to the normal coordinate system library and can select whatever you want. Normally, you use parametric transformation at this point.

OK, that was all. We go to Save. And you see at this point, we have both GSB files [INAUDIBLE]. Banking [INAUDIBLE] closed and can also close it here, and all was fine.

But we have also a look what we have to do when you need a transformation and why it works like this. [INAUDIBLE] I selected the right one, but the system [INAUDIBLE]. So I want a short view how to use.

To show you, I selected also the [INAUDIBLE] transformation path. That was the way where the transformation works. Before I do this, we first look at the projected coordinate systems.

No, OK. Here, that's the filter criteria for searching. And we find that every coordinate system was mentioned two times.

You see here and also nearly the same under it. But the second one has in the name the BeTA2007. [INAUDIBLE] 2007, but BeTA means the same.

At this point, you select when you assign the source coordinate system if it is using the parameter transformation or the field transformation. When you select the simple one, you will use a parametric transformation. When you want a field information, you use this.

[INAUDIBLE] a sign, and that was it. When you selected the BeTA one, let's have a look again on the geodetic transformation path. [INAUDIBLE] you.

So what you see is on top to down, the geodetic transformation used. And the first one you see at the top was the one we edit our GSB file. When this transformation will proceed, it will proceed the next one in the list of transformations.

And on this way, you see the source data here. That was the reason why this transformation path is used. So first, we find geodetic transformation.

You needed then define the path and the coordinate system using this path. OK, the next question need-- [INAUDIBLE]. Sorry, my PowerPoint don't want that guy.

How to produce our own GSB file-- also on this point, we will have a little bit life. Doing this, we change the drawing. In just drawing, we have an area.

Normal drawing, and we need now a GSB file for this area. What will we do? You have a little tool for generate a point grid inside of our [INAUDIBLE].

I follow the command. I'm seeing him how to save the file. At this point, he asked me if the file exists, I want to override it. And I do say yes.

Now we need the area. You can use the limits or the point in the left button and right top to define the area. Right side, you will see that is a good [INAUDIBLE].

We are using at this point 100 meter. And that's all, and we say, OK, create the file. He was successfully, and that was what we have.

Let's have a look what he produced. You see no point grid around our drawing, but he produced the file on the left side, the neutral point number, the x, and the y-coordinate rounded on our grid [INAUDIBLE]. With this file, you go to the official coming from government or company.

The program will officially transform to the new coordinate system. And this program you use may differ of the era where you live, what you want to do. So as a result, you get the second file with the same point numbers and the corresponding coordinate system.

When you have done this, you need the new command to be integrated in our tool. So it's this moment in German. You will translate it.

First, you have to select the coordinate system of the source. For this, you use the normal coordinate library [INAUDIBLE] in Civil 3D. And the short code for [INAUDIBLE] it.

Now I can give him the source points, the coordinate system for the target. [INAUDIBLE] 99 UTM and also [INAUDIBLE]. Now I select-- the target points are generated [INAUDIBLE] a different program. And I tell him, OK, where to find it.

The file exists at this point. So you will ask me again if I want to [INAUDIBLE] it. And last, my name.

When you select the source and the target file, you fill the table with the coordinates, find. And then, like I say, OK, [INAUDIBLE] NTv2 generated. At this point, I have done it [INAUDIBLE] all, and we get a new file.

It all works fine. With this generated GSB file, we can produce the same as we see above. No, first in the presentation.

OK, little look at this moment. Useful programs and links-- at first, when you're interesting in the Tools4GIS for generate a GSB file, you can use the first link. Or otherwise, the company KilletSoft has also some tools for NTv2 files. And also, they have a library with the whole world.

So have a look there, and you will find GIS e-files using this Civil 3D. If you have any question, you will now go to the time of Q&A after this presentation. You are welcome, and thanks for listening.