Why is there a difference in altitude between planned and recorded routes?
Different terrain models
As with all route portals, the calculation of elevation meters in our route planner is also based on a digital terrain model (DTM). This model is invisible under the map, regardless of which map basis is chosen.
How accurate or realistic the calculation of the elevation meters in a routing process is depends on the resolution of the DTM available in the respective region. The resolution is nothing else than the edge length of a square to which the same elevation value is assigned for the entire area. The smaller these squares are, the higher the resolution, i.e. the more accurate the DTM is. The larger the squares become, the coarser, i.e. less accurate the DTM is. In practice, this simplification of the real terrain inevitably leads to deviations - especially in the mountains and especially on ridges and peaks.
Currently our height service includes the following resolutions:
- Switzerland: 2 x 2 m
- Germany, Austria and Northern Italy: 10 x 10 m
- France: 25 x 25 m
- Rest of the world: 30 x 30 m
Due to technological advances in surveying technology, the available DTMs are becoming more and more accurate.
We are constantly looking for new, higher resolution models in order to be able to deliver even more accurate routing results.
Distance of the measuring points
Another factor that has a significant impact on the routing result is the distance between the individual measuring points (not to be confused with the manually set waypoints in the route planner!)
"Many measuring points = more accurate result" - Why this formula does not work out can be well illustrated by the so-called "elephant-ant problem": For an ant (short stride length) a curb is relevant, not so for an elephant. If all the curbs on a route were added together, the result would be a very high value, which is not true for an elephant - because it does not notice the curbs because of its longer stride length. With this problem, every routing provider has to decide whether to choose larger or smaller distances - We have deliberately chosen an average value because in practice, both too many and too few measurement points lead to a poor or unrealistic routing result. On a GPS device and also on some smartphones, however, it is possible to decide individually on a certain "step length".
So what is correct?
Differences also arise because there is no exact reference route. You might think "Why not record with GPS?!", but this data is not "more correct" than the digital terrain model. This has several reasons:
GPS is not exact - neither in altitude nor in distance. It always sets the measuring points slightly left or right of the path. On flat terrain this is not too bad, but on steep slopes or on the edge of a cliff, this results in meters of altitude that are not part of the route in reality.The path is identical in the satellite image and on the map and corresponds to reality (you can see this well on our hybrid map). The GPX track on the other hand is partly a bit off the track. So a recorded track does not always guarantee "perfect" data.
Our tip for the most realistic values
Always plan your route on our route planner - this way the data is not "right" or "wrong", but at least it is consistent with the other routes on our platform. Furthermore there are no deviations ("jumps") off the trail.
To know where you have been on the way during the postprocessing, it still makes sense to record the track via GPS device. To estimate how reliable the displayed data is, upload the track in our tour planner and get the aerial photo. If the track and the route in the aerial view are mostly identical, this indicates a high degree of accuracy.
