Implicit flow routing on terrains with applications to surface networks and drainage structures

Flow-related structures on terrains are defined in terms of paths of steepest descent (or ascent). A steepest descent path on a polyhedral terrain T with n vertices can have T(n^2) complexity. The watershed of a point p --- the set of points on T whose paths of steepest descent reach p --- can have complexity T(n^3). We present a technique for tracing a collection of n paths of steepest descent on T implicitly in O(n logn) time. We then derive O(n log n) time algorithms for: (i) computing for each local minimum p of T the triangles contained in the watershed of p and (ii) computing the surface network graph of T. We also present an O(n^2) time algorithm that computes the watershed area for each local minimum of T

Comparative field studies of various traps and attractants for the Mediterranean fruit fly Ceratitis capitata (Diptera: Tephritidae) in fall

Για την επιλογή της πλέον αποτελεσματικής παγίδας και ελκυστικού μεταξύ ορισμένων από τους τύπους παγίδων και ελκυστικών ουσιών που χρησιμοποιούνται σήμερα για την παρακολούθηση και καταπολέμηση της μύγας της Μεσογείου, Ceratitis capitata, (Wiedemann) (Diptera: Tephritidae), συγκρίθηκαν σε πορτοκαλεώνες επτά τύποι παγίδων (δύο τύπου MePhail, Υαλοπλαστική, χάρτινο-δίπτυχο, χάρτινος φάκελος-χαρτοπλαστική, διαφανής πλαστική φιάλη, παγίδα ξηρού τύπου), τέσσερα τροφικά ελκυστικά ( Dacus bait 100, Entomela 12SL, όξινο ανθρακικό αμμώνιο και το με κωδικό ZI), ένα ελκυστικό φύλου (Trimedlure) και ένας συνδυασμός τροφικού και ελκυστικού φύλου (Όξινο ανθρακικό αμμώνιο+Trimedlure). Τα αποτελέσματα έδειξαν ότι μεταξύ των παγίδων τύπου MePhail δεν υπάρχουν σημαντικές διαφορές.Η παγίδα τύπου πλαστικής φιάλης απεδείχθη πολύ αποτελεσματική όταν πληρούται με το ZI (πρωτεϊνικό τροφικό ελκυστικό). Από τα δοκιμασθέντα ελκυστικά , τα πλέον αποτελεσματικά ήταν το ZI και το Entomela. Ο συνδυασμός ελκυστικών τροφής και φύλου δεν έδειξε σημαντική συνεργιστική δράση στην αποτελεσματικότητα της παγίδας. Τα αποτελέσματα επιτρέπουν μια καλύτερη επιλογή μεταξύ των παραπάνω τύπων παγίδων και ελκυστικών, για παρακολούθηση και καταπολέμηση της μύγας της Μεσογείου.To clarify questions regarding the effectiveness of the many different types of traps and semiochemicals used for the monitoring and the control of the Mediterranean Fruit Fly, Ceratitis capitata (Wiedemann) (Diptera: Τephrititae), seven trap types, four food attractants, one sex attractant and a combination of food and sex attractant, were evaluated under field conditions in orange orchards in fall. No major differences were observed between trap designs resembling the original McPhail glass trap. The plastic bottle trap of 1,5L volume, with four side openings for one-way fly entrance, proved very efficient when filled with a proteinaceous food attractant Ζ1. From the attractants, two of them, Ζ1 and Entomela showed the best performance. The combination of food and a sex attractant showed no significant synergistic effects on trap efficiency. The findings allow a better choice among trap types and attractants, available today in the market, for Medfly monitoring and control

Analysis of flow and visibility on triangulated terrains

Landscapes and their morphology have been widely studied for predicting physical phenomena, such as floods or erosion, but also for planning human activities effectively, such as building prominent fortifications and watchtowers. Nowadays, the study of terrains is done in a computer-based environment; terrains are modelled by digital representations, and algorithms are used to simulate physical processes like water flow and to compute attributes like visibility from certain locations. In the current thesis we focus on designing new algorithms for computing structures related to water ow and visibility on digital terrain representations. Most specifically, the terrain representations that we considered are the so-called Triangulated Irregular Networks (tins), that is, piecewise linear surfaces that consist of triangles. One of the problems that are considered is the effect of noise on the worst-case complexity of visibility structures on tins. The view that a person can have from a point on the surface of a tin can be very complex, since in the worst case thin obstacles in the foreground may appear to fragment many long terrain edges in the background into visible and invisible pieces. In our analysis we considered tins whose triangles have some well-defined properties that terrains in practice are expected to have. Although complex visibility structures can be induced on such tins as well, we proved formally that slight perturbations on the elevations of the tin vertex set will always get rid of the high complexity. Another key problem that is studied is to design efficient algorithms that compute flow-related structures on tins. So far it was known that, in the case of tins, drainage structures that were computed using a consistent flow-model could have high complexity for specific input instances. We managed to develop a mechanism that can extract important information on flow paths and other drainage structures without computing those structures explicitly. This mechanism can be used as a basis for designing a variety of efficient algorithms, such as for computing the area measure of drainage structures or for computing structures that represent the terrain topology. The last part of the presented work involves the implementation of a software package that computes drainage structures on tins. In this package flow is modelled as following strictly the direction of steepest descent on the tin surface. Existing software for related applications either constrain flow on the edge set of the tin, or use inexact arithmetic, both of which introduces imprecise and/or incorrect results in the output. Our implementation is the first one that, at the same time, follows a robust flow model and uses exact arithmetic. We have used this implementation as a point of reference for evaluating experimentally the quality of the output of other flow models which are used in many hydrological applications. We have also used our software for conducting experiments on extracting watersheds on imprecise tins, that is, tins where the elevation values of the vertices are not exactly defined but are subject to noise from some given interval. Based on the results of these experiments, we have designed a novel method for extracting watersheds on imprecise terrains that produces high quality output

Analysis of flow and visibility on triangulated terrains

Landscapes and their morphology have been widely studied for predicting physical phenomena, such as floods or erosion, but also for planning human activities effectively, such as building prominent fortifications and watchtowers. Nowadays, the study of terrains is done in a computer-based environment; terrains are modelled by digital representations, and algorithms are used to simulate physical processes like water flow and to compute attributes like visibility from certain locations. In the current thesis we focus on designing new algorithms for computing structures related to water ow and visibility on digital terrain representations. Most specifically, the terrain representations that we considered are the so-called Triangulated Irregular Networks (tins), that is, piecewise linear surfaces that consist of triangles. One of the problems that are considered is the effect of noise on the worst-case complexity of visibility structures on tins. The view that a person can have from a point on the surface of a tin can be very complex, since in the worst case thin obstacles in the foreground may appear to fragment many long terrain edges in the background into visible and invisible pieces. In our analysis we considered tins whose triangles have some well-defined properties that terrains in practice are expected to have. Although complex visibility structures can be induced on such tins as well, we proved formally that slight perturbations on the elevations of the tin vertex set will always get rid of the high complexity. Another key problem that is studied is to design efficient algorithms that compute flow-related structures on tins. So far it was known that, in the case of tins, drainage structures that were computed using a consistent flow-model could have high complexity for specific input instances. We managed to develop a mechanism that can extract important information on flow paths and other drainage structures without computing those structures explicitly. This mechanism can be used as a basis for designing a variety of efficient algorithms, such as for computing the area measure of drainage structures or for computing structures that represent the terrain topology. The last part of the presented work involves the implementation of a software package that computes drainage structures on tins. In this package flow is modelled as following strictly the direction of steepest descent on the tin surface. Existing software for related applications either constrain flow on the edge set of the tin, or use inexact arithmetic, both of which introduces imprecise and/or incorrect results in the output. Our implementation is the first one that, at the same time, follows a robust flow model and uses exact arithmetic. We have used this implementation as a point of reference for evaluating experimentally the quality of the output of other flow models which are used in many hydrological applications. We have also used our software for conducting experiments on extracting watersheds on imprecise tins, that is, tins where the elevation values of the vertices are not exactly defined but are subject to noise from some given interval. Based on the results of these experiments, we have designed a novel method for extracting watersheds on imprecise terrains that produces high quality output

Exact and approximate computations of watersheds on triangulated terrains

The natural way of modeling water flow on a triangulated terrain is to make the fundamental assumption that water follows the direction of steepest descent (dsd). However, computing watersheds and other flow-related structures according to the dsd model in an exact manner is difficult: the dsd model implies that water does not necessarily follow terrain edges, which makes designing exact algorithms difficult and causes robustness problems when implementing them. As a result, existing software implementations for computing watersheds are inexact: they either assume a simplified flow model or they perform computations using inexact arithmetic, which leads to inexact and sometimes inconsistent results. We perform a detailed study of various issues concerning the exact or approximate computation of watersheds according to the dsd model. Our main contributions are the following. • We provide the first implementation that computes watersheds on triangulated terrains following strictly the dsd model and using exact arithmetic, and we experimentally investigate its computational cost. Our experiments show that the algorithm cannot handle large data sets effectively, due to the bit-sizes needed in the exact computations and the computation of an intermediate structure called the strip map. • Using our exact algorithm as a point of reference, we evaluate the quality of several existing heuristics for computing watersheds. We also investigate hybrid methods, which use heuristics in a first phase of the algorithm and exact computation in a second phase. The hybrid methods are almost as fast as the heuristics,but give significantly more accurate results. • We describe and theoretically analyze a new exact algorithm for computing watersheds, which avoids the computation of the strip map

Exact and approximate computations of watersheds on triangulated terrains

The natural way of modeling water flow on a triangulated terrain is to make the fundamental assumption that water follows the direction of steepest descent (dsd). However, computing watersheds and other flow-related structures according to the dsd model in an exact manner is difficult: the dsd model implies that water does not necessarily follow terrain edges, which makes designing exact algorithms difficult and causes robustness problems when implementing them. As a result, existing software implementations for computing watersheds are inexact: they either assume a simplified flow model or they perform computations using inexact arithmetic, which leads to inexact and sometimes inconsistent results. We perform a detailed study of various issues concerning the exact or approximate computation of watersheds according to the dsd model. Our main contributions are the following. • We provide the first implementation that computes watersheds on triangulated terrains following strictly the dsd model and using exact arithmetic, and we experimentally investigate its computational cost. Our experiments show that the algorithm cannot handle large data sets effectively, due to the bit-sizes needed in the exact computations and the computation of an intermediate structure called the strip map. • Using our exact algorithm as a point of reference, we evaluate the quality of several existing heuristics for computing watersheds. We also investigate hybrid methods, which use heuristics in a first phase of the algorithm and exact computation in a second phase. The hybrid methods are almost as fast as the heuristics,but give significantly more accurate results. • We describe and theoretically analyze a new exact algorithm for computing watersheds, which avoids the computation of the strip map

Fast generation of multiple resolution instances of raster data sets

In many GIS applications it is important to study the characteristics of a raster data set at multiple resolutions. Often this is done by generating several coarser resolution rasters from a fine resolution raster. In this paper we describe efficient algorithms for different variants of this problem. Given a raster G of vN × vN cells we first consider the problem of computing for every 2 = µ = vN a raster Gµ of vN/µ × vN/µ cells such that each cell of Gµ stores the average of the values of µ × µ cells of G. We describe an algorithm that solves this problem in T(N) time when the handled data fit in the main memory of the computer. We also provide two algorithms that solve this problem in external memory, that is when the input raster is larger than the main memory. The first external algorithm is very easy to implement and requires O(sort(N)) data block transfers from/to the external memory, and the second algorithm requires only O(scan(N)) transfers, where sort(N) and scan(N) are the number of transfers needed to sort and scan N elements, respectively. We also study a variant of the problem where instead of the full input raster we handle only a connected subregion of arbitrary shape. For this variant we describe an algorithm that runs in T(U log N) time in internal memory, where U is the size of the output. We show how this algorithm can be adapted to perform efficiently in the external memory using O(sort(U)) data transfers from the disk. We have also implemented two of the presented algorithms, the O(sort(N)) external memory algorithm for full rasters, and the internal memory algorithm that handles connected subregions, and we demonstrate their efficiency in practice

Implicit flow routing on terrains with applications to surface networks and drainage structures

Flow-related structures on terrains are defined in terms of paths of steepest descent (or ascent). A steepest descent path on a polyhedral terrain T with n vertices can have T(n^2) complexity. The watershed of a point p --- the set of points on T whose paths of steepest descent reach p --- can have complexity T(n^3). We present a technique for tracing a collection of n paths of steepest descent on T implicitly in O(n logn) time. We then derive O(n log n) time algorithms for: (i) computing for each local minimum p of T the triangles contained in the watershed of p and (ii) computing the surface network graph of T. We also present an O(n^2) time algorithm that computes the watershed area for each local minimum of T

Design of extensive green roofs for the major school plants of Piraeus

Despite the increase of global awareness regarding environment related issues and the development of relevant international policies; effective activities to conserve open space remain inconsistent in Mediterranean cities. Additionally as urban areas continue to expand and free space at street level becomes more and more limited there is a greater need for innovative green technologies which could contribute to the creation of sustainable urban ecosystems. Green roofs on buildings have already proved valuable for storm water management, energy conservation, microclimate mitigation, pollution remediation and biodiversity restoration, but their spreading in Mediterranean cities is still very limited. The shallow-substrate extensive type of green roofs is of great interest for massive application, which would lead to great scale positive effects in urban ecosystems. In Greece, in the framework of a relevant initiative called "Green roofs on public buildings", pilot extensive green roofs for the four major school plants of the city of Piraeus were designed. These green roofs which have a total area of about 0.6 ha are located in highly populated districts with negligible green spaces. This presentation is focusing on the decision making processes during the design and the selection of the growing medium and the planting material. The methods and the parameters of the relevant landscape, bioclimatic, hydrologic, irrigation and drainage studies are also presented. It is expected that the construction of these green roofs will improve the life quality of the school communities and more generally of the citizens of Piraeus

Implicit flow routing on triangulated terrains

Flow-related structures on terrains are defined in terms of paths of steepest descent (or ascent). A steepest descent path on a polyhedral terrain T with n vertices can have T(n2) complexity, since at worst case the path can cross T(n) triangles for T(n) times each. We present a technique for tracing a path of steepest descent on T in O(n log n) time implicitly, without computing all the intersection points of the path with the terrain triangles