The Arnales Peak, a three-thousander (Part One)
Just as in the Himalayas the kings are the eight-thousanders and in the Alps the four-thousanders, in the Pyrenees this select club is made up of the "tremiles". And no, it is not that below this symbolic figure there are no mountains with more character than many of those on the list(s) of three-thousanders, but it is that being a three-thousander has more "cache".
As you know, the number of three-thousand-metre peaks, those that comply with the rule of exceeding three thousand metres in altitude and having a prominence of more than 10 metres, is not definitive. However, in recent years the number has stabilised between approximately 212 and 217. How many are there exactly? We cannot yet say, but if there is a number to be given, the information published on the "Ghostbusters"This now leaves the figure at 216, after the inclusion of the Punta de la Brecha Superior de Llosás.
If you visit this website, you will see that just after the list of the three-thousanders there is a second list. In it you will find the peaks that at some point were catalogued as three-thousanders and later turned out not to be. Among them, a few aspirants are waiting for their case to be reviewed and claim "justice", such as Arnales peak, which has been removed from the list by 10 centimetres and which in some publications is included with a prior warning.
Well, with the new data available as of 22 February 2022, we will try to give a new opportunity to this peak located near the Panticosa Baths, or at least, to tighten the siege even more if possible.
The "Arnales" case
A first search on the internet quickly reveals the disparity in the figures for the altitude of this peak, ranging from the 3,006m listed by Buyse to the 2,996m shown on the IGN maps.
First check: visual survey of NGI maps
The first mandatory check is therefore to visually verify the altitude at which the Arnales appears on the maps of the official organisations. In the case of the IGN Iberpix viewer the figure that appears is still 2,996m, the same figure that appeared several years ago obtained by automatic stereocorrelation of the photogrammetric flight. A method that in areas with a lot of relief generally presents a greater error than in flat areas.
A second check of this same map (MTN25) was carried out by means of the CNIG Download Centre. As indicated, the 0145 sheet on which the Arnales peak is located is updated in March 2.021, however, it still does not offer us anything different from what appears in the viewer. A final check in the Iberpix viewer was carried out on the lidar layer, obtaining a result of 2,999m when the cursor was passed over the area.
Second check: SITAR maps
The second map checked by official bodies is the Topographic Map of Aragon at a scale of 1:5000 (MTA5). You can consult it in the viewer IDEARAGON. Here the Pico deros Arnals appears with an altitude of 3.000,00m.
A suspiciously round figure that invites us to think precisely that, in a rounding of the decimals, even more so when we see that the rest of the three-thousanders in the area appear with two decimals. The same figure appears in the Official Gazette of Aragon, in the document Order VMV/914/2020 approving in 2020 the last modification of the nomenclature of the Aragon three-thousanders of 2017.
Third check: Lidar DTM
Not being satisfied with the results obtained by studying the cartography of Aragon and the IGN, the next step is to go one step back to study the Digital Terrain Model (DTM). This is the previous product from which the relief for the corresponding topographic map is subsequently obtained. In this case, the sheet we are looking for is number 0145 corresponding to the National Topographic Map (MTN).
In the IDEARAGON we can download the DTMs for 2012 and 2015, while in the CNIG download centre the 2010 DTM can be downloaded. After the relevant checks, we found out that the 2015 DTM of Aragon is the same as the 2010 DTM of the IGN, corresponding to the first lidar coverage (2009-2015) of the National Aerial Orthophotography Plan (PNOA).
So we have two different DTMs (2012 and 2015 according to the Aragon nomenclature) generated from class 2 (terrain) of the lidar point cloud that we will see later.
To see it in a very graphic way, from the DTM we have generated contour lines at every metre. On the other hand, taking into account that the mesh pitch is 5 metres, we have taken a series of points in the Arnales area at that distance (taking every less would not make sense). The result can be seen in the following images:
While the MDT2012 offers a maximum height of 3000.92m, in the 2015 MDT the maximum height is 3000.15m. Unfortunately, we have not found any information on the technical specifications of the Aragon DTM2012. From the 2015 DTM, which is actually the 2010 flight of the PNOA, on the technical specifications published by the IGN, it is stated that the RMSE Z error of the DEM (the same for the DTM) is 50 centimetres.
In conclusion, according to what we have seen in the MDT2015 of Aragón (from the MDT of 2012 we lack information) we cannot state categorically that the Arnales peak is a 3,000m, as although it exceeds the 3,000m by 15 centimetres, the RMSE Z is 50 centimetres.
Fourth check: lidar point cloud
As with any derivative product, the generation of the DTM implies a loss of information. We can only resort to the most basic product publicly available, the lidar point cloud, which is distributed in 2kms x 2kms files and which, among other things, is already classified, the passes are stitched and the ellipsoidal heights are transformed into orthometric heights (geoid adapted to Spain EGM08-REDNAP).
The point cloud can be downloaded from the IDEARAGON or in the CNIG download centre. In both cases it is the same file and corresponds to the 2010 PNOA flight. In the case of Arnales Peak, only the first PNOA LIDAR coverage (2008-2015) is available. In other words, at least 0.5 points per square metre are guaranteed with an RMSE Z error that will generally be less than 40 centimetres and below 80 centimetres in 95% of the cases.
For the study we have kept only the points of the terrain class, which is used to generate the DTM. Zooming into the study area we have the following result:
This time, the result is more conclusive but not definitive. There are four points above 3000 metres (two of them 40 centimetres above) and a good number of points close to this figure. Therefore, the idea that the highest point is due to the fact that it has fallen on a large milestone or other type of human sign is ruled out, since between the first point and the fourth one above 3000 metres, there is a distance of more than three metres. In addition, it is not excluded that there are higher areas between each point, where we have no information.
Final conclusion:
Although it is not possible to state categorically that Arnales is higher than three thousand metres, in the lidar point cloud, the basis for obtaining altitudes in mapping, there are data that clearly indicate that it is a three thousand, being 3000.42m high.
A check that will be practically definitive in the future will be possible when the flight data from the second Lidar coverage, accurate to 20 centimetres, are published. In the meantime, the only option to resolve this issue definitively is to go up with a dual-frequency GNSS receiver and take measurements with an accuracy of less than 5 centimetres.
You may be interested in:
The Arnales (3.001,37m), definitely a three-thousander. A group of 5 surveyors called "Sostremetries" climbs to the top of the Arnales with a dual frequency GNSS receiver to determine the true height of this "new-old" three-thousander.
