Rock glacier velocity monitored by annual in-situ geodetic surveys: Long-term challenges, solutions and suggestions

We provide an overview of geodetic survey designs conducted on 43 rock glaciers (RG) in the European Alps, embed these in an overview of the pioneering phase of monitoring RG movement, and compare our approaches with other studies. Different in-situ geodetic approaches used for the annual survey of RG velocity (RGV) and RG frontal advance (RGA) are examined. Geodetic-based RGV monitoring in the Alps started in 1918 and RGA in 1921. RGV is an associated parameter of the Essential Climate Variable “Permafrost” since 2022. Nowadays, satellite-based positioning or classic measurements with a total station are used for in-situ geodetic surveys. The total number of surveyed boulders at the 43 RGs varies from 1 to 206. In most cases, only a part of these points is considered as “reference points” used to calculate the RGV value of RG units. The density of reference points for RGV calculation ranges from 27.5 to 1143 pts./km 2 . For RGV monitoring using annual geodetic surveys, we suggest: (a) selecting a RG where multi-decennial monitoring seems to be feasible, (b) distributing uniformly observation points over the entire RG unit of interest along either profiles or well-distributed boulders, (c) repeating measurements always at the same time of the year (September), (d) site-RGV-averaging using all data points, (e) site-RGV-averaging using only the survey markers considered to be the most suitable for long-term monitoring, (f) providing additional statistical information of these points and in particular of the central flowline, and (g) securing long-term storage of original data, metadata and results. • Geodetic surveying methods for 43 rock glaciers in the European Alps are examined • Overview of the pioneering phase of monitoring rock glacier movement is given • Monitoring began in 1918 for surface movement and in 1921 for frontal advance • Annual geodetic surveys use satellite-based positioning or total station procedures • Best practices for the annual geodetic monitoring of rock glaciers are proposed.