For my FKT run I followed the GPX track from the tour portal of the Harzer Tourismusverband e.V. (see https://www.harzinfo.de/erlebnisse/wandern/fernwanderungen-durch-den-harz/harzer-foersterstieg.html or https://regio.outdooractive.com/oar-harz/de/tour/fernwanderweg/harzer-foersterstieg/1538079/). This track shows the official course of the Harz Försterstieg, but shows slightly different start/finish points.
Officially, the forester's climb begins immediately before the Kaiserpfalz Goslar, in Google Maps there is a corresponding marker "Harzer Försterstieg". The GPS coordinates are 51.902851, 10.425774
Officially, the forester's climb ends at the "Kohlungsplatz" in Kamschlacken, a (parking) place at the northern end of Kamschlacken (crossing streets Große Söse/ B498). In Google Maps there is a corresponding marker "Harzer Försterstieg". The GPS coordinates are 51.764478, 10.393070
For my FKT run I used the official start/finish points - different from the track.
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