Geographic Information Systems Exam May 2014 PDF

SWEDISH UNIVERSITY OF AGRICULTURAL SCIENCES SLU TE0008, 5 HEC Department of Energy & Technology Geographic Information Systems Examiner: Dr. Tomas Thierfelder May 2014, 12:00 – 17:...

SWEDISH UNIVERSITY OF AGRICULTURAL SCIENCES SLU TE0008, 5 HEC Department of Energy & Technology Geographic Information Systems Examiner: Dr. Tomas Thierfelder May 2014, 12:00 – 17:00 Please label each page with the personal identification code acquired from the examiners assistant prior to the test. Please leave some space for correction marking. Please deliver your test to the examiners assistant when you’re through, or when test-time has passed. The test consists of ten questions where your answer will be ranked within the interval [0, 5]. Thus, the maximum sum of ranks is 50, whereof approximately 30 are required for passing the written exam. 1. In GIS, geographic objects are represented with raster and/or vector technique. Describe these two methodologies and how they may be used to construct the basic GIS-features points, lines and polygons. How are geo-references and attributes handled by each method? Make simple drawings and provide examples on real world phenomena that may be represented with the respective GIS-features. Vector data are designed for discrete objects and are represented with pairs of x, y coordinates. Point features have a single pair of coordinates and examples are a tree or an electric pole. Lines are represented with a start node and a stop node and usually several vertices between these nodes all having defined positions (pairs of x, y coordinates). Examples are roads or streams. Polygons are features with an outline built as a closed ring meaning the start node and the stop node of the outline share the same location. Examples are buildings or lakes. In vector data each feature has a set of attributes linked to it as a row in the attribute table with defined fields. Vector data can store topology (adjacency, containment) but are treated as being homogenous within each feature. Raster data are designed for continuous fields and are represented with a regularly tessellated grid of usually square cells. One location in the grid (e.g. the centre of the lower left cell) has a reference to real world coordinates. Each cell usually contains a single value. Raster data creates a complete coverage of the area and are thus good to use for describing elevation or temperature, but can also be used for discrete objects such as land cover classes. Point features are represented with a single cel data, line features as several adjacent cells and polygon features are represented with one or many cells depending on the cell size (raster resolution). Topology cannot be stored in raster format. 2. Present at least two advantages and two disadvantages with each method for representing geographic objects (raster, vector) in a GIS. Advantages with raster data are e.g. simple data structure, can store continuous surface data such as temperature, ability to represent fuzzy objects like wetland lakes and mountains. Examples of disadvantages are the lack of topology and their relatively large storage requirement. Advantages with vector data are e.g. exact geometry, ability to handle multiple attributes for each object, and a well-defined topology. Classical disadvantages are their unreasonable precision, their algorithmic complexity, and their inability to represent continuous phenomena. 1 3. Rasters and vectors may be considered as digitally implemented algorithms that smartly solve the fundamental GIS problem. Exactly what is this problem, and how is it solved in the two respective cases of rasters and vectors? The fundamental problem is to reduce the amount of information that is held in generic geography into an amount that may be managed by a computer. In raster technology, the amount of information used to represent reality is regulated by pixel size. In vector technology, generic information is reduced via weeding, e.g. by regulating the number of vertices used to represent a vector object. 4. The geoid is a rather strange surface that aligns to the WGS84 radius at average, but retains perpendicular to gravity at every location. This introduces local anomalies as compared with WGS84; how can this unevenness be utilized for the creation of a local geodetic datum? Please use the term “local ellipsoid” in your answer and illustrate it with a simple drawing. See handout lecture 3 for an answer. 5. The advantage of using a secant map projection, as compared with a tangential ditto, is easily expressed. What is it? Please provide an example on an international geodetic datum that utilizes a transversal secant cylinder as projection model, and where this cylinder is being rotated sixty steps in order to cover the circumference of the Earth. Lesser average scaling error. Universal Transverse Mercator (UTM). 6. Describe the difference between passive and active sensors for remote sensing. Provide a sensor example per technology, including the resulting map product. A passive sensor senses the reflection of external emission sources like the sun, whereas the active sensor contains an internal source of emission. The digital photocell is a passive sensor that is being used to produce orthophotos, whereas radar is an active sensor that is being used to produce images on pollution, algae bloom, and elevation. 7. What is ortho-correction good for, and how is it done? See handout lecture 5 for an answer. 8. When composing a map layout for presentation, what are the fundamental elements that should be included apart from the map itself? Provide at least five elements together with a short description of each. Title, legend, scale bar (and/or ratio), projection (and geographic datum), data source (copyright), north arrow (if needed), map grid (when using geographic coordinates), author and date, inset map. 2 9. What is a formal map, and how does it differ from a map-like visualization? What distinguishes a map from a chart? What distinguishes a reference map from a thematic ditto? Provide examples on the six map-types mentioned above. See handout lecture 6 for answers. 10. You’re employed by the national traffic authorities to plan the construction of a new highway in the east-west direction through mid-Sweden. The planning procedure is managed with a digital GIS; what would your main considerations be? What criteria would govern your choice of route? What data sources would you require? What overall GIS methodologies? What main obstacles would you anticipate? A large number of factors must be considered and weighted together; existing roads, urban centers, nature reserves, topography and hydrography, political strategies, etc., etc. All factors need to be superimposed on a map, and the new route sought in the resulting topology. The data sources should cover the decision-factors with thematic information, and be stored in a project map-base for storage and analysis. The main obstacles are probably political, where public opinions must be weighted into a comparison of the gains and losses associated with the new road. 3