Geographic Information Systems Exam (PDF) - March 24, 2015

SWEDISH UNIVERSITY OF AGRICULTURAL SCIENCES SLU TE0008, 5 HEC Department of Energy & Technology Geographic Information Systems Examiner: Dr. Tomas Thierfelder March 24 2015, 08:...

SWEDISH UNIVERSITY OF AGRICULTURAL SCIENCES SLU TE0008, 5 HEC Department of Energy & Technology Geographic Information Systems Examiner: Dr. Tomas Thierfelder March 24 2015, 08:00 – 12:00 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. Please define a datum of geographic information. Also define a geographic problem. Provide at least three examples of geographic problems. A geographic datum contains a position (given in a geodetic datum) together with an attribute value (that denotes the geographic content at the position). Problems that utilise geographic position for their solution are called geographic problems. Examples: Forest management, precision farming, finding your way in an unfamiliar city. 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. 3. A “spatiotemporal” scenario may be conceptualised as spatial change over a period of time. Can you suggest how to catch such change with a GIS? If you consider a time-series of temporal snapshots, one way of catching temporal change is via calculation of consecutive differences. This would result in a series of zero images if nothing has changed, and of non-zero values at the positions of eventual change (with the value reflecting the degree of change). 4. The geoid is a rather mysterious surface with a mean radius that coincides with WGS84, but with a surface that, at every position, is orthogonal to Earth’s gravity field (and is therefore irregularly shaped). How may the characteristics of the geoid be utilised when a local geodetic datum is defined? Please use the phrase “local ellipsoid” in your answer and illustrate with some extremely situated region (as compared with the global scenario). The geoid provides a local gravity reference as well as a smooth approximation of Earth’s surface. In order to catch the local gravity reference, a local ellipsoid is used to approximate the (local) curvature of the geoid. When objects situated at Earth’s surface are projected onto the local ellipsoid, the resulting surface is mathematically well-defined while the gravity reference as well as the relation to the global ellipsoid is maintained. 1 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. How many GPS technologies can you list? Please provide names and technological characteristic. The maximum number of technologies is five. What is the benefit of real-time GPS as compared with GPS that does not operate in real-time? What is the main benefit of a GPS that does not operate in real-time? In consecutive order: absolute code in real-time (absolute GPS); relative code in real-time (differential GPS); relative phase-shift with post-calculation (statistical GPS); relative phase-shift in real-time (single station RTK); compensated relative phase-shift in real-time (network RTK). Real-time GPS operates while being moved around, which may be utilised for navigation. Non real- time GPS (statistical GPS) requires stationarity for a long enough period of time to calculate a highly accurate position (post-centimetre precision). 7. What is ortho-correction good for, and how is it done? With most sensors being close to point-shaped in comparison with the area of land that they sense, a slight shift of perspective is introduced in the sensed image. The reason for this is the shifting distance between sensor and the horizontal plane below, where the distance is minimal in zenith and increases towards the peripheral image. The result is a convex image that may be ortho- corrected via projection onto a concave surface. The ortho-corrected image does not contain any shift of perspective, and is hence “seen from zenith” regardless the position. The problem with shifting perspective across the image decreases with the distance between sensor and Earth. 8. What is the difference between a formal map and a map-like depiction? What is the difference between maps and charts? What is the difference between reference maps and thematic maps? Provide an example regarding each of the six map-types mentioned above. Formal maps are being produced in accordance with cartographic conventions and are being used as geographic reference. One example is the Swedish GSD terrain-map in scale 1:50,000. Map-like depictions are being used to visualise, analyse, and editing geographic information without any formal claims at all. One example are the cartograms available in most subways where the system of trails and stations are depicted without formal geo-positioning (formality is traded for pedagogy). Maps are referring to terrestrial depiction whereas charts are referring to maritime areas of application. The GSD terrain-map exemplifies the former, maritime navigational charts the latter. Reference maps depict the land with the attributes required for recognition, whereas thematic maps focus on specific (thematic) attributes. Again the GSD terrain-map exemplifies the former, whereas the extent of malaria throughout western Sudan exemplifies the latter. All maps are situated on a continuous scale in-between purely reference and purely thematic. 2 9. Provide a brief account for the principles that govern invers-distance-weighted interpolation. In the invers-distance-weighted context, what is an isotropic interpolation kernel? Most interpolation methods rest on the assumption that a continuous field has been observed in a finite number of localities, and that some spatial correlation structure may be utilized to restore continuity. In the case of inverse distance weighing, the value of the interpolation kernel is determined by a linear combination of all surrounding observations weighted inversely proportional to their distance (from the kernel). In isotropic interpolation, the observations are weighted irrespective their angular direction (from the kernel), whereas isotropic aninterpolation assumes a direction-dependent interpolation kernel. 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