This app is able to handle offline height data from ".hgt" files in various situations. These height data will be added to all points. Especially they are useful in case of route planning, since you get immediately in idea about the accumulated height gain of the route and as well they are the basis for the visualisation of the height profile of a route.
Since gain/loss calculation based on GPS data is very faulty, this calculation prefers meanwhile hgt data over gps data (relevant, if device has no barometer).
This private project provides data, mostly based on lidar technology. For details see the page of the Sonny. This app currently using the Europe v22 data from Sonny. To make this data available to the app, there is a copy of it on a github hgt data project.
The ESA provides hgt data created by the SRTM (Shuttle Radar Topography Mission) project. Simply go to http(s)://step.esa.int/auxdata/dem/SRTMGL1/ , all files like "N54E014.SRTMGL1.hgt.zip" are available for HTTP download.
After you assign a newly downloaded map to a visible layer, you will be asked whether to download corresponding height data. If you confirm the download, that's all.
Alternatively you may use the hgt grid layer that provides an easy overview about the local available height data. The Section Downloading of missing hgt data describes the download of missing hgt data in detail.
The automatic download prefers data from Sonny, and takes ESA data as fallback. If ever you do an manual download of height data, place these downloaded hgt files as they are in the directory "MGMapViewer/hgt" (do not unzip them).
Here is some more information for developer.