047] Global Mapper - Correcting DEM Layer Elevation: Alter Elevation Values

While it doesn't happen often, there are rare cases where you need to correct the elevation of DEM (Digital Elevation Model) data. This is necessary when you want to emphasize something from a design perspective by maximizing the elevation difference in a flat contour.

"Before we begin, if you missed the previous session, check it out here:"

[012] Terrain Information Lines: Calculate Elevation/Slope Stats

[046] Parallel Line Offset: Create Parallel Lines Spaced from Selected Line Feature(s)

In Global Mapper, there are methods to correct DEM data by either adjusting the entire layer or correcting only a specific area.

Today, I will cover how to correct the entire layer. It is very simple.

First, load the DEM data. I will skip the process of creating a DEM as I have already introduced it in previous lectures.

[img 2 : Initial DEM data loaded in Global Mapper]

Now, select the DEM layer in the Control Center.

[img 3 : Selecting Options for the DEM layer in the Control Center]

Select 'Options'. (You can also simply double-click the layer).

The Layer Options window will appear.

[img 4 : Alter Elevation Values tab in the Elevation Options window]

Understanding the Alter Elevation Formula

The most important part is the formula shown in the red box. The correction is applied according to this equation:

new_elev = (scale Factor * orig_elev²) + offset

(Remember the order of operations for basic arithmetic!)

If you want to double the entire DEM elevation value, enter '2' in the Scale Factor and click apply. A location that was 1m high will become 2m, and a location that was 10m high will become 20m.

If you want to raise the entire DEM by 5m, enter '5' in the Offset and click apply. A 1m elevation will become 6m, and a 10m elevation will become 15m.

If you want to raise the entire DEM exponentially, enter '2' in Power and apply it. A 1m elevation remains 1m, but a 10m elevation becomes 100m.

What if you want to double the total height and then increase it by 10m? You would enter '2' for the Scale Factor and '10' for the Offset. An area that was 1m will become 12m, and a 10m area will become 30m.

It's easy, right? Try entering values and check if the results come out as expected. You can verify this by displaying the elevation legend.

In atlases, we sometimes see 3D images where the contours of an entire country are applied. If you use the actual elevation as is, it often looks flat. Therefore, the elevation is corrected overall so that mountain ranges are expressed more prominently.

Valid Elevation Range

Next, let's look at the Valid Elevation Range.

• Maximum Valid: The highest elevation to be permitted.
• Minimum Valid Elevation: The lowest elevation to be permitted.

The current elevation has a maximum of 545 and a minimum of 120. Let's adjust this.

[img 5 : Setting the Maximum Valid elevation to 400 meters]

Elevation areas exceeding the maximum will be deleted. Strictly speaking, they are hidden. Don't click 'OK' yet; just click 'Apply' to see how it changes.

First, let's look at the current status.

[img 6 : Original DEM status before applying elevation range limits]

Now, if we set the maximum to 400 and click apply:

[img 7 : DEM after applying Maximum Valid 400 - areas above 400m are hidden]

The parts exceeding 400m have disappeared.

You can leave these empty areas as they are, or you can fill them with a specific value by entering a number in 'Replace Invalid With'.

[img 8 : Filling hidden areas with 200m using the Replace Invalid With option]

If you enter 200 and click apply, the disappeared areas are filled with a 200m elevation.

The 'Clamp Out of Bound Values to Valid Range' option applies the maximum or minimum value to parts that exceed those ranges.

If you set the maximum to 400 and the minimum to 200, and then apply 'Clamp Out of Bound Values to Valid Range':

[img 9 : DEM with Clamp Out of Bound Values applied - all values fixed between 200m and 400m]

You can see that the entire elevation legend has also shrunk accordingly.

Restoring to Original

To return to the original terrain:

[img 10 : The Restore Default Elevation Range button]

Just click 'Restore Default Elevation Range'. You can return to the initial DEM at any time, so feel free to experiment with the correction values.

I didn't show the elevation legend at first, but to verify how the elevation range actually changes, make sure to display the legend while working.

That's all for today!

"In the next session, we'll dive deeper into more advanced features. Stay tuned!"

[048] DEM Partial Elevation Correction: Terrain Paint Options