AGH UST Land Surveyors Verify Peak Heights. Why Do the Measurements Change?

On the left side, there is a GNSS receiver. It is the colour of steel and stands on a tripod. The AGH UST flag is wrapped around the tripod. The flag is composed of three horizontal stripes coloured, from the top, green, black, and red. The background of the photo constitutes a mountain crest. Granite rocks covered in green flora are visible. Above the mountain crest there is a blue sky, which is partly covered by white and grey clouds.

Specialised GNSS receiver, measuring heights based on satellite measurements, from the materials of the “Setka w Koronie” project

Scientists from the Faculty of Mining Surveying and Environmental Engineering have measured the heights of the highest summits in Ukraine and the Crown of Polish Mountains. Their efforts, which have already been reported in the media, have been described in several scientific articles.

Kebnekaise is the highest mountain in Sweden, and it has two peaks. Until recently, its southern peak was deemed the highest point (Sydtoppen). However, according to the measurements conducted in 2018 by researchers from Stockholm University, Sydtoppen had to renounce its primacy in favour of its northern sister (Nordtoppen), which crests up to 2096.8 metres above sea level. Interestingly, it was global warming that deprived Sydtoppen of its top position. The southern peak is covered in an ice cap, which has melted in the last 50 years by about 24 m. Nordtoppen, in turn, is entirely rocky.

Kebnekaise is counted as the Crown of European Mountains, that is, a list of the highest peaks on the Old Continent, and simultaneously constitutes a prestigious challenge for mountain enthusiasts who attempt to climb all summits. Those enthusiasts who had put their feet on Sydtoppen might want to reconsider whether they actually visited the roof of Sweden. Importantly, as little as a dozen-centimetre difference in height measurements can have a significant impact on tourism, enforcing, for instance, corrections in terms of trail layouts. This is often met with disapproval on the part of local communities, which can suffer substantial losses due to the presumptive changes.

‘An interesting example is the Khardung La pass in the Himalayas, where the difference between the height proposed by the local authorities and the actual height of the pass is 242.7 m. Indians believe that this is the highest pass available to motor vehicles in the world. They claim that the pass crests to 5602 metres above sea level, whereas, in fact, it is 5359.3 metres above sea level. Where does it come from? It boosts the tourist attractiveness of this place and influences the country’s economy, which means that everything boils down to money. Such examples abound on the world scale, including Poland which was the testing ground for our research’, explain Dsc Kamil Maciuk from the Faculty of Mining Surveying and Environmental Engineering and Dr Michał Apollo from the Institute of Geography of the Pedagogical University in Krakow, the co-authors of the article published in the prestigious Current Issues in Tourism journal.

‘An inaccurate measurement of a peak’s height can have a significant influence on its popularity, especially if it relies on being the highest. Therefore, it is important to measure again those mountains that are often visited by tourists with the help of modern technology, so that cartographic disturbances can be avoided’, says Professor Yana Wengel from Hainan University in China, who also participated in the preparation of the article mentioned above.

False heights on maps

Insofar as the most famous peaks in the world are constantly being remeasured with the help of cutting-edge measuring techniques, unfortunately, those less popular ones do not enjoy an equal amount of attention. Maps or other materials used by tourists are still full of numbers which have been determined dozens of years ago, using archaic methods, or numbers flawed in other ways.

To change such flaws has become the aspiration of scientists from the Faculty of Mining Surveying and Environmental Engineering. In cooperation with Vasyl Stefanyk Precarpathian National University in Ukraine, they have verified the height of Mount Hoverla, the highest peak in Ukraine (current measurement – 2055.5 m above sea level) and Pip Ivan (2019.4 m above sea level). In measuring these peaks, the scientists have found several-metre differences in relation to the measurements that were published to date. DSc Jacek Kudrys, Assoc. Professor Małgorzata Buśko, Assoc. Professor Krystian Kozioł, and Assoc. Professor Kamil Maciuk have shared the results of their measurements in the pages of Maejo International Journal of Science and Technology.

The project “Pomiar i weryfikacja Korony Gór Polski” [Measurement and verification of the Crown of Polish Mountains] has reverberated through the media. For the 100 years of the AGH UST, the scientists measured the highest peaks of all mountain ranges in Poland, taking into account the new division into mesoregions introduced in 2018. The verification has not only shown occasional substantial differences in heights (actual and published), but has dethroned some peaks that were part of this popular tourist list of mountains. ‘These are a kind of nuances that arise from the methods of choosing the highest point and measuring it, but also from the evaluative approach to the current data’, explains Professor Krystian Kozioł who, together with Dr Maciuk, described the measurements in the Remote Sensing journal.

Measuring height using modern methods

Let us have a look at those nuances which constitute the focal point of problems related to measuring heights of summits. The team’s measurements were conducted using the GNSS technology (Global Navigation Satellite System), based on satellite-aided measurements of heights. Currently, this is the most precise, yet very time-consuming, method of measurement, as it requires scientists to climb the mountain and reach the peak. To measure the height, the specialised GNSS receiver must be placed at the highest point on a mountain. This instrument allows scientists to achieve much more precise measurements than in the case of smartphones or smartwatches equipped with GNSS modules.

Comparison of the height of the Turbacz peak, measured by mobile GNSS receivers, from the materials of the “Setka w Koronie” project

Frequently, the peak had been forest-covered or unavailable, which prevented the scientists from performing measurements at the highest spot. In such cases, the receiver was placed as close to the peak as possible in a spot with a clear horizon, and the difference in heights was calculated using geometric levelling.

Professor Kozioł says that those research escapades were frequently comical: ‘I will tell you about a situation we had on the Postawna peak in the Eastern Sudetes. The peak looks as follows: there is a flat, watery terrain, a ‘marsh’ with lush flora. So when we were looking for the highest point, our shoes and trousers were wringing wet, but we have eaten a lot of blueberries’, recounts Professor Kozioł smilingly. The adapted methodology allowed the AGH UST scientists to achieve the accuracy of measurements with a margin of error of one decimetre. ‘I strongly encourage you to read all of our publications’, says the scientist.

K. Maciuk, M. Apollo, J. Mostowska, T. Lepeška, M. Poklar, T. Noszczyk, P. Kroh, A. Krawczyk, Ł. Borowski, P. Pavlovčič-Prešeren, Altitude on Cartographic Materials and Its Correction According to New Measurement Techniques, Remote Sens. 13

S. Szombara, M. Róg, K. Kozioł, K. Maciuk, B. Skorupa, J. Kudrys, T. Lepeška, M. Apollo, The Highest Peaks of the Mountains: Comparing the Use of GNSS, LiDAR Point Clouds, DTMs, Databases, Maps, and Historical Sources, Energies. 14 (2021)

All results were cross-referenced with data

from the ALS (Airborne Laser Scanning), gathered in the government ISOK system. The data had been collected from the entire territory of Poland during flights of airplanes equipped with laser rangefinders. The device emits a laser beam to a specified target, measuring the distance based on the time in which the reflected beam returns back to the rangefinder. On the basis of the findings of such instruments, a very dense cloud of points is created, described with coordinates, determining the location of the points on the ground and their height. Next, based on this cloud, DTMs (Digital Terrain Models) and DEMs (Digital Elevation Models) are created.

The analysis showed that such data cannot be approached uncritically: ‘We have to realise that the laser beam from the airplane could not have hit the ground, but, for instance, vegetation of various heights or other objects on a given peak. If such points are not filtered out correctly, they might be taken into account when creating the digital terrain model. The results of the project show certain discrepancies between the data from the ISOK system and our measurements. The numbers were not always larger or smaller, but oscillated in relation to the point which had been taken into consideration during interpolation. On the Turbacz peak, for instance, a bench was chosen as the highest terrain point by the ISOK system (acc. to the coordinates), while the one we have found is located near the concrete base of an obelisk standing on the peak’, describes Professor Kozioł.

Historically speaking...

There is a different explanation for the discrepancies between our modern measurements and the historical data, the majority of which dates back to the second half of the 19th century. ‘As far as old maps are concerned, the measurement method had the greatest influence on the discrepancies. In the past, technology was different, not as precise and accurate as today. According to the current land surveying standards, the ground is what we base our measurements on. It was not always so in the past. For instance, on the peak of Śnieżnik mountain, there is a field of debris from an old object, which, when the previous measurements were taken, was most probably marked as the highest point’, describes Professor Kozioł.

Apart from the fact that the measuring standards have changed significantly (information that was not always easy or possible to find), the discrepancies also resulted from, among other things, the use of other reference points. The maps show the true altitude of peaks, i.e. expressed in height above sea level. However, the point is that this “sea level” is not as fixed as one might think. For several years in Poland, the reference point has been the averaged yearly measurements of the tide gauge on the North Sea in Amsterdam (the Netherlands). Earlier, it had been the tide gauge that indicated the level of the Baltic Sea in Kronstadt (Russia). In the years 1918-1939 and on the territory of the Prussian Partition, “level zero” was indicated by the point in Amsterdam; however, in the Austrian Partition, it was the point in Triest and the level of the Adriatic Sea. As far as the difference between Kronstadt and Amsterdam is not colossal, as it is merely 14 cm, in the case of Kronostadt-Triest it amounts to 48 cm. Therefore, from the heights of peaks in the Beskid Mountains measured before the year 1918 you have to subtract around half a metre to begin with!

Elevation conundrum

Currently, we still have different reference systems defining “sea level”. In Europe, the discrepancies can reach up to 2.3 m. This is the difference between the sea levels used in the Dutch Amsterdam and the Belgian Ostend. Is there a possibility to compare the heights of mountains measured in different reference systems?

Land surveyors using the GNSS technology have no problems with that as the measurements pertain to the global model of ellipsoid of revolution. This intricate term hides the mathematical surface that is most accurately adjusted to the averaged levels of seas and oceans – a geoid. The latter is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is also mathematically extended under the continents. In this case, the distribution of mass on the surface of the geoid is uneven, making it difficult to precisely determine it. That is why scientists also use a model of reference ellipsoid determined without having regard to the influence of distribution of topographic masses – a quasi-geoid. ‘At sea level, the geoid and the quasi-geoid are practically aligned. In high mountains, the differences may reach up to a few metres’, specifies Dr Jacek Kudrys from the Faculty of Mining Surveying and Environmental Engineering.

The differences between the level of the quasi-geoid and geoid in the world, acc. to the EGM2008 model. The colour scale of the differences is expressed in metres (Source: icgem.gfz-potsdam.de/home)

The “above sea level” height, which can be found on maps, is given either in relation to the geoid – orthometric height, or, as it is in Poland, in relation to the quasi-geoid – normal height. Currently in Poland, the quasi-geoid model pertaining to the averaged sea level in Kronstadt is applied.

Thus, when we want to compare the ellipsoidal height to the sea level, we have to consider the distance between the ellipsoid and geoid or quasi-geoid.