Determining Optimal Locations for Viewpoints Using the Open-Source Whitebox GAT Software

The most visually exposed landscape can be determined by a rich set of GIS tools, the main limitation of which is high intensity of computation. This study aims to put forward a method of specifying optimal location for the viewpoints attractive to tourists by means of Whitebox GAT, an open-source GIS application. The study area involves the Kolbudzko-Przywidzka Upland of the southern part of the Kashubian Lakeland in Poland. The method presented herein is characterized by simplicity and low computation intensity. However, it can only be used to analyse views on a local scale, in areas whose spatial coverage does not exceed a dozen or so kilometres

On IO-efficient viewshed algorithms and their accuracy

Given a terrain T and a point v, the viewshed or visibility map of v is the set of points in T that are visible from v. To decide whether a point p is visible one needs to interpolate the elevation of the terrain along the line-of-sight (LOS) vp. Existing viewshed algorithms differ widely in which and how many points they chose to interpolate, how many lines-of-sight they consider, and how they interpolate the terrain. These choices crucially affect the running time and accuracy of the algorithms. In this paper our goal was to obtain an IO-efficient algorithm that computes the viewshed on a grid terrain with as much accuracy as possible given the resolution of the data. We describe two algorithms which are based on computing and merging horizons, and we prove that the complexity of horizons on a grid of n points is O(n), improving on the general O(na(n)) bound on triangulated terrains. Our finding is that, in practice, horizons on grids are significantly smaller than their theoretical worst case bound, which makes horizon-based approaches very fast. To measure the differences between viewsheds computed with various algorithms we implement an error metric that averages differences over a large number of viewsheds computed from a set of viewpoints with topological significance, like valleys and ridges. Using this metric we compare our current approach, Van Kreveld's model used in our previous work [7], the algorithm of Ferreira et al. [6], and the viewshed module r.los in the open source GIS GRASS

PARALLEL VISIBILITY AND FRESNEL-ZONES CALCULATION USING GRAPHICS PROCESSING UNITS

Delo opisuje inovativno metodo izračuna vidnosti [61, 62] in Fresnelovih con na digitalnih zemljevidih z uporabo grafično procesnih kartic CUDA NVIDIA. Izdelani so trije vzporedni algoritmi: • modificiran vzporedni algoritem R2 za računanje vidnosti (R2-P), • algoritem za izračun zakrivanj Fresnelovih con (FZC), • algoritem za izračun prečnega preseka Fresnelovih con med oddajnikom in sprejemnikom (FZTI). Na osnovi uveljavljenega sekvenčnega algoritma R2 za računanje vidnosti je razvit modificiran vzporedni algoritem R2-P, ki za pohitritev izračuna poleg večnitenja izkorišča še druge uporabne lastnosti grafične procesne enote. Združen dostop do globalnega pomnilnika pripomore k hitrejšemu pretoku podatkov in s tem k hitrejšemu izračunu. Izmenjava informacij med nitmi v času računanja igra ključno vlogo pri pohitritvi. Izračun vidnosti na poljubno velikih podatkih je omogočeno s segmentacijo digitalnega zemljevida. Modificiran vzporedni algoritem R2 je primerjan z že implementiranimi algoritmi za izračun vidnosti v smislu točnosti izračuna in časa izračuna. Izkaže se, da je novi algoritem enako točen kot že uveljavljeni sekvenčni algoritem R2, hkrati pa omogoča bistveno pohitritev izračuna. Čas izračuna je skrajšan iz reda nekaj minut na red nekaj sekund. To pa v praksi pomeni možnost interaktivnega dela. Pri načrtovanju radijskega pokrivanja je poleg vidnosti zelo uporaben podatek o zakrivanju Fresnelovih con. Pri algoritmu za izračun zakrivanj Fresnelovih con se izbere lokacijo radijskega oddajnika, višino oddajnika, opazovano višino sprejemnika nad terenom in valovno dolžino radijskega valovanja. Algoritem za vsako točko terena izračuna, katera Fresnelova cona je zakrita. Rezultat je digitalni zemljevid z izrisanimi območji zakrivanj Fresnelovih con, kar o radijskem signalu na terenu pove precej več kot izračun vidnosti. Predvsem na področjih, kjer je prva Fresnelova cona povsem zakrita, se v primerjavi z izračunom vidnosti pridobi v praksi zelo uporabna informacija. Algoritem ima tudi možnost upoštevanja rabe tal, kjer se višina terena poveča v odvisnosti od rabe tal (npr. za gozdno površino reda 15 m). Z modifikacijami, kot sta vpeljava Friisove enačbe in upoštevanje smernega diagrama anten, postane algoritem enostaven propagacijski model in tako primeren za izračun radijskega pokrivanja. Izračun radijskega signala se primerja z izmerjenimi vrednostmi na terenu za frekvence 90 Mhz (FM), 800 MHz (LTE) in 1800 MHz (LTE). Za različne vhodne parametre enostavnega propagacijskega modela se izračuna standardna deviacija sprememb med izmerjenimi in izračunanimi vrednostmi in se jih prikaže na grafih. Tako se pridobijo najbolj optimalne vrednosti vhodnih parametrov za vsako frekvenčno področje posebej. Algoritem za izračun prečnega preseka Fresnelovih con med oddajnikom in sprejemnikom izračuna sliko Fresnelovih con, ki predstavlja matematični presek vseh skaliranih prečnih presekov Fresnelovih con vzdolž radijske poti. Rezultat je vizualna slika, ki pokaže lastnosti radijske (linkovske) zveze v smislu zakritja posameznih Fresnelovih con. V praksi bi algoritem najbolj koristil pri načrtovanju radijskih linkov, kjer bi lahko preverili, koliko in kateri del Fresnelovih con manjka zaradi ovir (terena). Vsi trije algoritmi so implementirani kot moduli GRASS GIS in se lahko uporabljajo na vsakem osebnem računalniku, ki ima vgrajeno grafično procesno enoto CUDA NVIDIA in naloženo ustrezno prosto dostopno programsko opremo.The work describes an innovative method with which to calculate the visibility [61, 62] and Fresnel zones on digital maps using graphics processing NVIDIA CUDA cards. Three parallel algorithms were formulated: • modified R2 parallel algorithm for calculating visibility (R2-P), • algorithm for calculating Fresnel zone clearance (FZC), • algorithm for calculating Fresnel zone transverse intersection between the transmitter and the receiver (FZTI). The R2 parallel algorithm was developed based on the established R2 sequential algorithm for computing visibility. Aside from threading, other useful features of the graphics processing unit were used to speed up calculation time. Coalesced access to the global memory helps speed up the flow of information and thus also speeds up the calculation. Exchange of information between threads during computation plays a key role in the speedup. The segmentation of the digital map enables the calculation of visibility for huge data sets. The modified parallel R2 algorithm was compared with the already implemented algorithms for the viewshed calculation in term of accuracy and duration of the calculation. It turned out that the new algorithm R2-P had the same accuracy as the already established sequential algorithm R2, although the former also makes it possible to significantly speed up the calculation. Calculation time is reduced from the order of a few minutes to the order of a couple of seconds. This, in practice, means that there is a possibility of interactive work. In addition to the viewshed, Fresnel zone clearance is very useful for planning the radio coverage. Algorithm FZC starts with the location of the radio transmitter, the height of the transmitter, the receiver observation height above terrain, and the wavelength of the radio waves. The algorithm for each point of the terrain calculates the first clear Fresnel zone. The result is a digital map with the plotted areas of Fresnel zone clearance. This map provides better information about the radio signal than just a calculation of the viewshed. Indeed areas where the first Fresnel zone is completely obscured are particularly good for providing very useful information. The algorithm also has the ability to take into account land use, where the height of the terrain is raised as a function of land use (eg. For the forest area, raising can be 15 m). With modifications, such as the introduction of the Friis transmission equation and consideration of the radiation pattern, the algorithm becomes a simple radio propagation model and thus is suitable for the calculation of radio coverage. Calculation of the radio propagation is compared with the measured values on a field for frequencies of 90 MHz (FM), 800 MHz (LTE) and 1800 MHz (LTE). For a variety of input parameters, the standard deviation of changes between the field measurements and calculated propagation is presented in graphs. In this way, the optimal values of the input parameters for each frequency band can be obtained. The algorithm for calculating Fresnel zone transverse intersection between the transmitter and the receiver produces an image of Fresnel zones, which represents the mathematical section of all scale cross-sectional Fresnel zones along the transmission path. The result is a visual image that shows the characteristics of the radio link in terms of masking individual Fresnel zones. In practice, the algorithm is most useful in the design of radio links, where man can check how much and which part of the Fresnel zone is missing due to terrain obstacles. All three algorithms were implemented as GRASS GIS modules and can be used on any PC with an integrated GPU NVIDIA CUDA and loaded with the appropriate free-access software

Towards Optimal Line of Sight Coverage

Maintaining the line of sight to a moving object or person over long distances is critical in many applications, e.g., mobile communications, security, surveillance. Determining the best places to position (or build) technologies is difficult because even small changes in the location can greatly affect the so-called viewshed, which is the collection of land areas within line of sight of a given observer. The need for multiple sensors or towers further complicates this problem, as they often need to work cooperatively to achieve the best possible coverage. This study proposes a novel approach that consists of three separate inventions: 1) An algorithm for calculating viewsheds from many sensors in parallel, 2) Introduction of a meaningful measure of quality for coverage to compare competing configurations; and 3) Optimization of that well-defined objective function to find the best suitable sensor parameters for practical applications. Preliminary results suggest unprecedented performance on a wide range of real terrains