Documentation: Tools - Adjust Altitude

Tool: Adjust Altitude

There are 3 types of devices to measure the altitude:

Device with barometer
Device with GPS
Device with GPS and barometer

Each of these devices has some difficulties to measure the altitude correctly.

Even a GPS device with barometric measurement, like the Garmin Edge 705, has errors. When this device is switched on, it can take several minutes until the device has adjusting the altitude to the correct value. When you don't want to wait so long and just start immediately, it can take more than one hour until the device has adjusted the altitude.

This tool provides the possibility to adjust the altitude for all kinds of errors.

Content shortcuts
Open adjustment tool
Adjust altitude without GPS data
Adjust altitude with GPS data

Available Adjustment types:
Approach with SRTM data
Replace with SRTM data
Set START and END to the same altitude
Adjust Max altitude
Adjust END altitude
Adjust altitude by distributing evenly

Open Adjustment Tool

This tool can be opened with the action Adjust Altitude... at several locations either in the context menu or directly in the toolbar (in the tour editor).

Adjust altitude without GPS data

None GPS devices can't use the SRTM data to adjust the altitude but the following adjustment types are available for these devices:

Set START and END to the same altitude

Adjust Max altitude

Adjust END altitude

Adjust altitude by distributing evenly

Adjust altitude with GPS data

When the recorded data contain GPS positions, there are two additional possibilities to adjust the altitude:

Approach altitude with SRTM data
Replace altitude with SRTM data

SRTM data are not perfect and have a tolerance of several meters but they give a raw estimation for the correct elevation.

The other available adjustment types can also be used to set the altitude data to the correct values.

GPS example with SRTM data

Displayed graphs in this tour chart example:

red line:
elevation from SRTM data
green graph:
altitude from the device. The device is adjusting the altitude over time but I couldn't figure out the algorithm.

Display SRTM data

When the adjust altitude dialog is opened, the SRTM data will be displayed automatically when GPS data are available.

To display SRTM data outside of this dialog, select Show SRMT Data in the chart options menu.

Approach altitude with SRTM data

This adjustment type is using the original altitude values from the device and approaches them to the SRTM elevation. When a GPS device with barometric measurement is used, this is the preferrable method because the device data are more accurate compared with the SRTM elevation.

These are the actions which can be done in this adjustment type:

Move left vertical slider

When the left slider is moved to the right side, the altitude is adjusted with linear interpolition. The problem is, that in most cases, the linear interpolition does not fit the recorded altitude to the SRTM elevation. To solve this problem the linear interpolition can be adjusted with splines and the spline points which are the big points on the blue line.

The left slider must be moved from the start point to the right side to see the spline points.

Create Spline Points

Spline points can be created at the mouse position by opening the context menu (right mouse button) and select Create Point. The mouse must be positioned between the start position of the tour and the left slider.

Move Spline Point

When the mouse is moved over a spline point, the mouse pointer shows the Resize All shape (four arrwos). When pressing the left mouse button and moving the mouse, the point will move and the spline is adjusted immediatedly.

Delete Spline Point

Spline point can be deleted by moving the point either before the start position or to the right side of the left slider. The button Remove Points will delete all points except one.

Chart graphs

red line	SRTM elevation
black line	difference (with absolute values) between the SRTM elevation (red line) and the saved (or updated) altitude (green area)
red filled area	adjusted altitude graph, left slider must be moved to the right side to see it
green filled area	altitude graph which is currently saved for this tour when the action Update Altitude is done it displays the updated altitude
brown filled area	this is the red and the green area, because the two colors are painted over each other and each color have a transparency, the resulting color is brown

Spline

blue line	spline which start and ends at the blue points
blue points	static spline points which can moved only vertical
red points	spline points which can be moved

Scenario to adjust altitude with Splines

Left slider is at the start position, this is the default position.
Left slider is moved to the 6km position
Left slider is moved to the 19km position
Red point is moved down to adjust the recorded altitude to the SRTM elevation
Additional points are created and positioned to adjust the altitude more precisely
Recorded altitude is adjusted with 4 spline points, this results in a black line which is approaching the 0 value
The action Update Altitude displays the altitude graph with the adjustments and resets the vertical position of the Spline points. The action Save Modified Tour will make the tour adjustments permanent.

Replace altitude with SRTM data

This will replace the altitude value of the recorded tour with the SRTM elevation values.

In the tour example, the peaks in the middle of the tour do not exist and are wrongly displayed because the elevation in the SRTM data are not always precisely.

Set START and END to the same altitude

Start and max altitude can be manually adjusted.

End altitude is automatically adjusted to the start altitude.

Adjust MAX altitude

Maximum altitude can be manually adjusted.

The start or end altitude is also adjusted according to the profile.

Adjust END altitude

End altitude can be manually adjusted.

The start altitude will keep the elevation and is not adjusted.

Adjust altitude by distributing evenly

The altitude for the whole tour can be adjusted manually.