Original Article Date: Oct 1, 2004

This tutorial shows how you can create a digital terrain model from elevation data without the need for add-on software. We'll then go through the process of how you can drape an aerial photograph over the resulting DTM using only the tools found in MicroStation. Way cool!

2004-10-12 Updated to correct misinformation regarding the image origin.
2005-07-29 Update to add a link.

For information pertaining to MicroStation V8i, please refer to this article:

[[Make a DTM and drape an image MicroStation V8i]]

 

Thank You!
With many thanks to the following folks who contributed to this article:

Jorge Ramis for providing and posting a spanish translation of this article.
Refer to: http://www.geocities.com/jorgeramis/Ask_Inga_Make_DTM.htm

Ron Jones from The WSDOT Design visualization Group for spoting an error and providing the correct information for an update to the article.


Peter Bekkers and Rudi Wells for providing the idea by posting a question on the 3D discussion group.

Greg Owen from Universal Geosystems for generously providing the dataset and for challenging me to make this article more than I had intended to. :-)

The aerial photograph was purchased from Tarin Resource Services, an industry leader in providing quality aerial photo products.

The DEM data used to process the elevations was obtained from AltaLIS, providers of accurate digital mapping data for Alberta.

Also thanks to Bentley's Dave Assaf and Ron Jones from The WSDOT Design Visualization Group for providing additional technical content and support.


Explore the file:
Download, open and explore 220_Points.dgn. In this 3D file you'll notice a model called Points that contains zero-length lines representing the peaks and valleys of a surface or ground terrain. Although these points are supplied with this tutorial, you can easily create your own if you have design software that can export DTM data to an XYZ text file. Once you have a text file, you can import the data into a design file by using the Import Coordinates tool. 
Navigate and open Points as the active model.

Create the DTM:
To create a triangluated network of these points, follow these very easy steps:

Create a new level - you might want to call it DTM for Digital Terrain Model.
Make this your active level, and set the active colour to 0.
Put all the points in a selection set.

Key in mdl load facet;facet dialog
You will be rewarded with the following dialog:

This little known dialog has been around for a while and is absolutly full of tools designed for creating slick 3D surfaces including a DTM from points, cells, or text. Since we're going to create a digital terrain model from the points, navigate to the XY Points tab and turn on Expand to Rectangle to draw a bounding rectangle at the lowest elevation.  Then select the Triangulate XY Points button to process the mesh. 

 Enable Expand to Rectangle and enter a datapoint to process. 

To clearly see what the Expand to Rectangle button does, delete the resulting mesh element, and regenerate the triangles with the switch turned off.

The difference is obvious.

Note:  You can also use this combination keyin: mdl load facet;facet triangulate xypoints. This will produce the same results, but eliminates the need for the opening the dialog. Note that this keyin will use the current on/off status of the Expand to Rectangle switch.

During the generation process, MicroStation will also open it's little text window and display three values prepended by the following letters:
V - The number of verticies in the resulting element.
F - The number of faces or triangles in the resulting element.
C - The number of connected output mesh elements. For the triangulation process, this value should always be 1.

Adjust the global lighting:
At this time the mesh is ready to be rendered 'as is' or you can drape an aerial over it which I'll explain in the following pages. In the meantime, let's just render the triangles to get a better idea of what the terrain looks like.

Before proceeding, ensure that you have generated the mesh without the bounding rectangle as explained above. You'll also want to turn off the elevation points since they are no longer needed.

In order to get a better rendering from this particular model, we'll first adjust the global lighting.

Select Tools > Visualization > Rendering > Global Lighting and in the resulting dialog adjust the settings so they match the graphic below.

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Render the surface:
Select the Render tool from the Render Tools toolbox and adjust the settings as follows:
Target = View, Render Mode = Smooth, and Shading Type = Normal.
Enter a datapoint on the isometric view and admire your results.

Sneak peak and tweek of the aerial:
The next step in this tutorial is to drape an aerial photo over the resulting digital terrain model. However, before we do that, let's take a sneak peek at the aerial by simply attaching it via the Raster Manager.

From the Raster Manager, select File > Attach and select 220_Image.jpg. This image is georeferenced, so be sure to turn off Place Interactivly from the attach dialog.

To remove the black collar around the image, double-click the image entry in the Raster Manager and navigate to the Colour tab. Turn on Transparent and set the Transparent Color to 0. Finally, poke on the Apply button.

Gather necessary information for draping the aerial:
Before going on to the draping process, you will need to grab some information from the image.

Pop back to the attachment settings of the image via the Raster Manager. Navigate to the Location tab and note the Dimensions, Pixel Size, and Origin of the image. It's a good idea to write these values down on a scrap of paper since you'll be needing them in a bit.

Dimensions - These values are the distances along the X and Y axis expressed in master units. The sample image measures 5286 meters in the width, and 5228 meters in the height.

Pixel Size - This is the measured distance in master units of one pixel in the image. Our image has a pixel size of 1 meter.

Origin - This is the XY location of the lower left corner of the image. So, the lower left corner of the image is positioned at XY=378864.5, 5993712.5

When images are applied using Elevation Drape as the mapping method, they are tiled so that all four corners are coincidental. This means that each corner of the image is adjacent to the other three corners as shown below. (With thanks to Ron Jones from the The WSDOT Design Visualization Group for this bit of information and for the graphic below.)

Draping introduction:
Ok...so now we are ready to get to the draping process. The ideal and preferred software to use for draping is Bentley Descartes which drapes images in a snap and requires no effort from the user. However, for those of you who don't have Bentley Descartes you can still drape images by following along with the remainder of this article.

I'd like to point out that the draping workflow was originally published in an 1998 MicroStation Manager Magazine article called Draping photos over MicroStation models authored by Bill Hanson. Bill does a wonderful job of describing the theory behind each step and I recommend that you read his article to supplement the following abridged version.

Whenever you're ready....
Detach the image and display only the DTM

Draping Workflow Overview:
Create a material table to store material assignments in the file.
Create a palette file to store material characteristics.
Create a new material to point to the image.
Assign the aerial photograph to the new material.
Adjust the material characteristics.
Assign the material to the DTM
Render the view.

Create material table and palette file:
From the Rendering Tools tool box, select Define Materials and you will be presented by a rather large dialog. This dialog is the "control center" for palettes and their material definitions.

When you first access this dialog, MicroStation will populate the left side with an entry that is the same as your file name. This entry is the beginnings of a material table which is a file with a .mat extension. A material table stores material assignments to elements in the design file that live on specified levels and have specific colour.

From the menu bar, select Palette > New
MicroStation responds by adding New Palette (1) under the material table.
Rename this to PhotoDrape by selecting Palette > Save As, or by right-clicking the entry and selecting Save As from the picklist.

In doing so, MicroStation creates a palette file which has the extension of .pal.

A palette is a simple text-based file that contains a materials characteristics. To see what I mean, you can go ahead and open a delivered palette file from .../bentley/Workspace/system/materials/. You'll also notice that these files make reference to the "patterns" which are the actual JPG images. These images are in the .../Workspace/system/materials/pattern/ folder. Finally, palette files also refer to "bump maps" which adds roughness or texture to the pattern. They are typically greyscale contrasting or companion images that have the letter "b" as the last character. These will be found in the .../Workspace/system/materials/bump/ folder.

Create a new material:
Select the New Material button and rename New Material (1) to Aerial

Assign the aerial as the material:
Select the little icon highlighted in the graphic below and select 120_Image.jpg.

Once the image has been loaded, you will be presented with the following dialog which allows you to customize the settings or characteristics for the material.

Adjust the materials characteristics:
This step will require you to add the information we gathered about the image. Hopefully you still have those numbers near by!

Adjust the settings to be as follows:
Mapping to Elevation Drape

X Size = 5286 and Y Size = 5228
This is the width and height of the image in master units and was obtained via the Raster Manager in a previous step.

Offset X = 378864.5 and Offset Y = 5993712.5
These are the lower left coordinates of the image and were grabbed from the attachment settings in the Raster Manager.

Close pattern dialog and save your changes by poking on the Save button in the Material Editor dialog.

Hint: When you have unsaved work, the entries on the left side of the Material Editor will be blue. When there have been no changes, and there is no need to save, the entries will be black. See the graphic below.

Dismiss the Material Editor dialog and select Apply Material from the Rendering Tools tool box.

Double check that you have the correct palette and material selected as shown in the graphic below.

Poke on Assign by Level/Colour and select the mesh element representing the terrain.

Render the DTM:
From the Rendering Tool tool box, select the Render tool and adjust the settings as follows:
Target = View, Render Mode = Smooth, and Shading Type = Normal.

Enter a datapoint on the isometric view and admire your results.

As you can see, there's a fair bit of work involved in draping an image over a DTM using plain old ordinary MicroStation. If you're going to be doing a fair bit of this, I recommend that you take a second look at MicroStation Descartes which eliminates 95% of the work.

Other than that, I hope you've found this a worthwhile tutorial and that you'll be able to produce some fine looking DTM's in no time at all.

AskInga Article #220