Block 2 - Understanding GIS Flashcards
Is a paper site map of the headquarters of the Open Universiteit in Heerlen a descriptive or a predictive model?
It is a descriptive model, because it provides a static description of the university grounds.
Explain the difference between a descriptive and a predicitve model.
A descriptive model describes a more or less static situation and a predictive model describes the process to predict future situations
Explain when and where descriptive modelling in GIS is needed.
Descriptive modelling is necessary if you want to make a two- or three-dimensional representation of a person or thing, typically on a smaller scale. In geography, you typically make a 2D representation of the earth’s surface (or things located on top or under the earth’s surface) and store it in your computer. So, the data stored in your computer are a descriptive model of the real world. You can use your database to create a map and print this map on paper. This paper map is another descriptive model of the real world. So, you need a descriptive model to set up a spatial database of your study area in GIS. Sometimes, somebody has already done this for you, e.g. if you use the topographic map of the geographical survey (Dutch: topografische dienst). Anyway, you should realize that in setting up a spatial database, explicit and implicit assumptions and choices are made about what to include in the database, how to store it and how to represent it. These assumptions and choices can have an important impact on the usefulness of your database. For example, if you decide to ignore spatial phenomena which are smaller than 1 x 1 meter, this determines the spatial resolution of your information. Adding this information in a later phase of the project, can be problematic. It is therefore very important that the development of the spatial database is a deliberate process.
What is the difference between a paper map, a digital map and a GIS map?
The main difference is the flexibility. A paper map is much less flexible than a digital or GIS map. A digital or GIS map can be printed or processed using GIS technology. In this perspective, a digital or GIS map is preferable over a paper map.
The difference between a digital map and a GIS map is the number of layers. A digital map generally consists of one layer, whereas a GIS map can consist of multiple layers.
After a sand excavation close to the riverbed of the river Rhine, the area needs to be reconstructed in an economically valuable way. One of the options is to create a recreational area for swimming during summer and ice skating during winter. First an inventory has to take place whether there are enough potential users of such a recreational area. Which kind of maps do you need to get this insight?
At least maps with population density and transportation routes, which you can use to make an estimate of how many people may visit the area in relation to travelling time. You would also need some general data about the amount of swimmers and skaters in the population.
Decide whether the following phenomena are spatially discrete or continuous:
- The concentration of Zinc (a heavy metal) in the soil.
- The average annual rainfall.
- The provinces and all major cities in The Netherlands.
- The major transport routes from Rotterdam harbour to the destination of the goods.
- The spatial distribution of mice in a nature area.
- The concentration of Zinc (a heavy metal) in the soil: continuous.
- The average annual rainfall: continuous.
- The provinces and all major cities in The Netherlands: discrete (provinces and cities are discrete administrative areas).
- The major transport routes from Rotterdam harbour to the destination of the goods: discrete (roads and other transport routes are discrete objects).
- The spatial distribution of mice in a nature area: discrete. The mice themselves are discrete entities, although they are generally not individually represented on a map. It is more common to use mice density, which is a spatially continuous parameter.
Describe the main difference between classification and aggregation.
Classification is the ordering of entities (objects) into classes that have common characteristics. It is stated as follows in the syllabus: “A thematic classification implies that if a phenomenon meets a predefined set of characteristics or class attributes, then it is member of that specific thematic class”. Aggregation is the joining of two distinct spatial objects into one spatial entity.
The difference between classification and aggregation can best be illustrated in an example. Imagine a GIS modeller that wants to make a road map. The modeller has several options to represent a road with two bike lanes on the map:
- Distinguish a separate entity class for roads and a separate entity class for bike lanes. In this case, a road with two bike lanes will result in three spatial entities being stored, i.e. one road and two bike lanes.
- Distinguish one common entity class for roads with and without bike lanes. Only one spatial entity is being stored in the database (i.e., the road), but the bike lanes can be stored as attributes of the road. It is not possible to show the bike lanes separately on a map (because the bike lanes have not been stored as separate spatial entities), but it is still possible to check whether a road has bike lanes. This is a typical example of aggregation: the information about the bike lanes is aggregated with that of the road.
- Distinguish one common entity class: “roads”. The information about the bike lanes is ignored and not included in the database (e.g., because it is considered irrelevant).
While aggregating, you are actually joining together more than 1 spatial entity into 1 spatial entity. And while classifying, every spatial entity remains apart, but they are labelled the same if they belong to the same class. The number of classes and the level of aggregation are a matter of choice and depend on the scale or level of spatial resolution that is needed.
Describe the relationship between scale and aggregation in your own words.
The smaller the scale, the more you will aggregate
Explain the importance of map projections for users of GIS.
A map projections is method that is being used to project a 3D object (like the globe of the world) on a 2D surface. There are different types of map projects, e.g. a cylindrical, azimuthal or conic projection. Knowing the map projection of a map is important for a user of GIS because:
- When overlaying maps (data analysis), these maps need to be in the same projection;
- You need to know the map projection in order to translate map coordinates into geographic co-ordinates.
Why is a conic projection often used for European maps?
Projections always have distortions. These distortions are smallest in the areas where the projection plane touches the earth’s surface. Assuming that you want to show the North pole in the upper part of the 2D map, a cone will touch the earth’s surface around Europe (cylindrical and an azimuthal projections touch the earth’s surface around the equator). Consequently,distortion for Europe is smallest using a conic projection.
What is the difference between projections and reference systems?
A projection determines how a 3D surface (e.g. the earth’s surface) is projected on a 2D surface. A reference system determines the location of an object (vector entity or raster cell) in relation to other objects or to the earth.
What is the disadvantage of a rectangular co-ordinate system such as the UK Ordnance Survey’s National Grid? How is this disadvantage related to the size of the mapped area?
In a rectangular co-ordinate system, a map graticule (or grid) is put over the projected 2D map. The coordinates of this system are being used torefer to locations on the map. Since a projected 2D map of the earth’s surface always has distortion (small distortions for small projected areas; large distortions for large projected areas), the distance between two points of the map does not always accurately reflect the real distance. For example, 1 cm at the meridian on a map with a cylindrical projectiwill present a larger distance than 1 cm at the North or the South pole.
What is the difference between a rectangular and a geographic co-ordinate system?
In a rectangular co-ordinate system the coordinates refer to a location on the map, whereas in a geographic co-ordinate system the coordinates refer directly to a location on the earth’s surface. So, in a geographic co-ordinate system, the coordinate system is put over the earth’s surface and then projected (together with the earth’s surface) on a 2D plane. In a rectangular co-ordinate system, the earth’s surface is first projected on a 2D plane and then a rectangular co-ordinate system is put over the 2D plane.
What kind of reference system is the ZIP code system which is used by postal services?
It is a non-co-ordinate system.
What kind of spatial referencing systems are being used by GPS devices?
A GPS uses signals from GPS satellites to work out the exact location of the user on the earth’s surface. As a consequence, a GPS will essentially use a geographical co-ordinate system. Most modern GPSs contain different projection algorithms, enabling the translation of geographic co-ordinates into a multitude of different (geographical and rectangular) co-ordinate systems.
