DEM

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A DEM (Digital Elevation Model) contains points with elevation data. DEM Data are based on LIDAR (Light Detection and Ranging) technology measurement, also known as Airborne Laser Scanning. There are DEM with point data arranged in a regular grid with a constant distance between the points. This distance is called cell size. Other DEM contain data points arranged irregularly (cloud-model).

Read more about this topic: http://en.wikipedia.org/wiki/Digital_elevation_model

A laser beam splits as it hits objects. The result are multiple returns. The difference between first and last return can show object height. The last return doesn’t always reach the ground.

Source: Lohani, Bharat. Airborne Altimetric LiDAR: Principle, Data Collection, Processing and Applications.

DEM Import Wizard

Read more about importing DEM file on the page DEM Import Wizard.

Open

Open an OCAD DEM file (*.ocdDem). OCAD 12 can also open ocdDems created in OCAD 11.

The OCAD DEM file is a DEM grid in an OCAD internal file format created in the DEM Import Wizard. This file is optimized for fast access to the height values.

For more information (e.g. cell size) about this file open the DEM Information dialog.

Show Frame

Shows blue rectangle with the extent of the loaded DEM.

When moving the mouse cursor inside the frame the height values is show in the status bar together with the coordinate.

Resize

Resize the loaded OCAD DEM file (make a subset) and save it as a new OCAD DEM file or overwrite the existing one.

Info

Shows information about OCAD DEM file.

When moving the mouse cursor over the file name then the file name with path appears.

Close

This function closes OCAD DEM file.

Merge DEM

Choose Merge from DEM menu to merge two different DEMs.

DEM 1: Choose the first DEM.
DEM 2: Choose the second DEM.

The two DEMs must have the same cell size.

Calculate DEM Difference

Choose Calculate DEM Difference from DEM menu. The Calculate DEM Difference dialog box appears.

Usually it is the difference between a terrain model and a surface model.

Add Upper DEM = DSM data file
Add Lower DEM = DTM data file
Click OK.

In the DEM Import Wizard it is possible to import DTM and DSM and create the DEM difference in one step.

To visualize the DEM difference choose Classify Vegetation Height

The extent of the Upper DEM and Lower DEM can be different. OCAD takes the overlapping area for the new DEM.

The cell size of Upper DEM and Lower DEM can be different. OCAD takes the cell size of Upper DEM for the new DEM.

Example

This is an example to show what can result from the Calculate DEM Difference function.

This is a DTM (Digital Terrain Model) with a cell size of 5m shown as hypsometric map:

The next picture shows the DSM (Digital Surface Model) of the same area with a cell size of 2m as hypsometric map. The buildings (northern part) and forest (south western part) are slightly visible.

The next picture shows a Difference DEM with a cell size of 2m shown as raster background map after calculating the DEM Difference. In addition, heights were colored using the Classify Vegetation Height function.

The area with no difference of the DTM and the DSM are displayed white.
A height difference up to 15m appears red.
The greater the difference, the greener an area appears.

When moving the mouse cursor over the map the difference is shown in the Status Bar.

Data source: Test data Wabern from swisstopo.