A lot of people do not distinguish between maps and reality. Maps are nor reality, but a model of it. They same accounts for GIS. The way data is selected for GIS, processed in a GIS analysis and presented in maps, can heavily influence both the results and even more so the impression, which the final output - in most of the cases a map - makes on somebody who looks at it. The data which should model a part of reality can be distorted, fragmented, neglected, denied, exaggerated, or linked to certain associations. As a lot of GIS products are used as decision making tools, one has to be extremely aware of what has been done to the data and how the data is presented. Without such awareness, one has to believe more or less blindly what one sees. Maps as well as a lot of GIS processes are not something objective and trustworthy, but rather something very subjective and suspicious. In 1991, a  book came on the market, bearing the title "how to lie with maps" (Monmonier 1991). This book revealed a number of remarkable tricks that had been used by map producers to misguide or influence the opinion of the map reader.  Everybody who works with GIS should be aware of such methods, both as a producer of GIS products and as a consumer.
 
 

Issues of Data Input

• Satellite images provided by companies, especially cheap/free products like quickshots, may be incorrectly georeferenced or show a different area than the company says.
• The resolution of a satellite image should be chosen according to the scale/spatial characteristics of the object of study
• A common fault for false results is the lack or misuse of metadata, like the  age of the data, the projection or the datum. Data with different datum can cause for example incorrect results in overlays. When georeferencing areas, especially very small areas, one has to be aware of the datum in which ground control points are given on hardcopy maps which are used as the reference.
• Statisitical Data should be carefully checked (MAUP, statistical assumptions: See the talk about MAUP and Spatial Statistics)

• Most classifications are subjective, man made models. One has to take care of the relation between reality and the classification (F.e., if an area with less than 10% Vegetation is classified as "desert", it may - depending on the resolution - still have patches of forest or grassland). Especially boolean classifications are less reliable for many real world objects than methods used in fuzzy logic. Be aware of the default button!
• All interpolation methods have to be chosen according to the nature of the object one wants to interpolate. Interpolation methods which are not appropriate may cause many problems. (See talk about Kriging)
• Statistical issues can cause major misrepresentations of real phenomena (See talk about MAUP). In a lot of cases, statistical problems remain unsolvable.
• Awareness of causes for error&uncertainty (See the talk about Error&Uncertainty by the Geographer's Craft)
• The Area of Study is always a critical point. For example, a study which shows how well a community is provided by a service, does not show services which are just outside the area of study - so the edges of the area of study tend to be not very well represented.

• Projections can cause very different impressions about the size of objects. Objects with the same size on a map may have different sizes in reality. For example, Mercator projections exaggerate the size of areas near the poles - Greenland looks as big as South America (Real proportion: 1:8). Such maps have been used for political "propaganda", for example, the exaggeration of the size of the former Soviet Union. Be aware when looking at maps which are used for weather forecast: Low or high pressure systems which seem to have equal size on a map are in reality often bigger/smaller in proportion to each other.
• Thematic maps often do not show spatial proportions - For example, a customer between two providers of goods will not choose the best provider according to distance (not shown on map), but according on other impressions made by the map.
• Generalizations may hide features which the producer of a map does not want to show. Generalizations also tend to make false impressions of street junctions (difficult junctions are often simplified), sizes of objects (all generalizations are "lies") and position of objects to each other.
• Colour: Colours cause strong impressions, for example red is associated with danger, alarm, heat and so on whereas blue is associated with water, cold etc. Two maps (which show the same features) with different colours can cause very different emotions: Environmental Groups like Greenpeace tend to give clearcuts red, yellow or another not "natural" colour (which means that the clearcuts are seen as artificial, alarming and disturbing) while forest companies give clearcuts on their maps greenish colours.
• Labels: There are a lot of choices to label one and the same feature. For example, clearcuts can be called "young forest" or "newly planted area". The same is due to maps which show political claims (Some Pakistani maps show the area of Kashmere as claimed by Pakistan, Indian maps show it as claimed by india)

• Monmonier, Mark: How to lie with Maps. University of Chicago Press. Chicago 1991
• Monmonier, Mark; George A. Schnell: Map Appreciation. Prentice-Hall. New York 1988
• For Interpolation: Lam, Nina: Spatial Interpolation Methods. American Cartographer 1983. 10(2): 129-149
• For MAUP: Openshaw S; Alvandies S:  Applying geocomputation to the analysis of spatial distributions.  In Longley P, Goodchild M, Maguire D,
• Rhind, D (eds) Geograhpic Information Systems: Principles and Technical Issues. Vol 1, 2nd ed. New York: John Wiley and Sons Inc.1999
• For Cartography: Robinson, A. H.; Morrison, J. L.; Muehrcke, P. C.; Kimerling, A. J.; Guptill, S. C. : Elements of Cartography. New York. John Wiley & Sons. 1995
• See also the list of links on the Geog470 page: http://www.geog.ubc.ca/courses/klink/gis.notes/resources.html#cartography
• For Error&Uncertainty: http://www.colorado.Edu/geography/gcraft/notes/error/error.html