## 1 Introduction  
This article is dedicated to the geodesist, polar explorer, and friend [PERSON]. He was the project initiator and scientific leader for a long time. The article is also based on his results, so he is mentioned here in memory as an additional author (Figure 1). The processes and consequences of climate change have been discussed for many years. Leaving aside catastrophic changes such as volcanic eruptions or earthquakes,  
Figure 1: Project initiator [PERSON] (1959–2019) on his last Greenland Expedition in 2017.  
it is indisputable that never in the recent history of the Earth, i.e., in the last million years, have there been fundamentally very rapid changes in the living conditions on our planet [1]. However, the climate today is changing rapidly. Geodetic measurements can make important and precise contributions to the monitoring of changes. In addition to remote sensing technique, which uses a wide variety of technologies, there are also ground-based measurements. These serve as \"ground truth\" for remote sensing, but because of their accuracy, they can also be used independently. In the 1970s, only ground-based measurements, often obtained during scientific expeditions or from measuring stations, could be used to monitor the Greenland ice sheet. Aerial methods were also used, but only for coastal parts of Greenland. Here it is worth recalling the pioneering expeditions that began exploring inland Greenland more than a century ago. One of the first was certainly the expedition of [PERSON] (1888-1889), who was the first to cross the southern part of Greenland on skis [2]. He brought back a wealth of scientific information and meteorological measurements and proved that the entire Greenland interior was covered by an ice sheet. Another important expedition was undertaken by the Swiss [PERSON] in 1912. [PERSON], a meteorologist, crossed Greenland with three other expedition members considerably further north than [PERSON] [3]. He was shortly followed by [PERSON] and [PERSON] [4; 5]. After the First World War, expeditions were more frequent and much better prepared technically. Airplanes also began to be used for research after the First World War. The systematic mapping of the coastline by the Danish Geodetic Survey in 1931-1934 was significant. The mapping work was carried out using photogrammetry from an aircraft. Today there are thousands of unique photographs in the Danish Airbase project database, which serve as a source of information on the historical state of glaciation [6; 7]. Several US military airfields were built in Greenland during World War II, some of which were converted to civilian airfields after the war and are still in use today [8]. Germany also built a small meteorological base in Greenland, but it was destroyed by an American air raid [9]. After the war, economic development began in Greenland, but other US military bases were also built in Greenland during the Cold War. More intensive research on the Greenland ice sheet took place after the fall of the Iron Curtain in the 1990s. Modern instruments, expedition equipment, and technical support were available, as well as the possibility of using satellite data. The significant progression of global warming and the rapid melting of the western and southern parts of the Greenland ice sheet, in particular, increased interest in research activities [10; 11; 12]. Combined data sources and non-traditional technologies like drones, for example, were used in research. Today, drones are the most popular, which allow very detailed measurements in smaller areas, e.g., tracking the movement of a glacier face or capturing the surface with cm resolution [13; 14; 15]. Special remote sensing satellites have been used for a long time, since the 1970s, but it's only relatively recently that some data has been free of charge and freely downloadable.