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One of the inherent properties of a GIS is its ability to perform spatial queries; that is, its ability to investigate spatial relationships among objects. Such objects can be virtually anything with a geographic location, including health centres, water towers, traffic signals, road intersections, and rainfall catchment. Because each object can be geometrically described as a point, line or polygon, we can ask questions such as "What is the shortest route from home to work?," "Which new homes have yet to be connected to the sewer system?," and "Where is the nearest sports complex that has a squash facility?". ArcView provides several spatial analysis tools to answer such questions and to examine and solve more complex problems. We shall deal with very simple, straight forward queries and use ArcView as the means to help us. It is more important to understand the spatial problem presented, then it is to remember which combination of ArcView buttons to press to solve the problem.
The bell has rung and another school day begins for Mrs. Hikmat's Grade 2 students. Her first task each morning is to take attendance, 24 present in the classroom and 3 absent with the flu. Perhaps without even realizing, Mrs. Hikmat has just answered a spatial question, "Which students are present today in my classroom?". In GIS terms this question would be reduced to a very common point-in-polygon problem and could be restated as follows, "Identify all points within a particular polygon." In this case, the points would represent students and the polygon would represent Mrs. Hikmat's classroom. Either a point is inside the polygon or is not inside the polygon. Similarly, a student is either present in the classroom or is not present in the classroom.
Speaking of students and classrooms, the Al-Meeri family has just moved to a new neighborhood and want their 7 year old daughter Sarah to resume studies at her new school right away. Mrs. Al-Meeri has heard about the excellent reputation of the school at which Mrs. Hikmat teaches, and so wishes to send her daughter there. She is informed by the Ministry of Education that because of the bus routing system, children can only attend schools in the same school catchment zone in which they live. Therefore, if the Al-Meeris live in school catchment zone A and the school of choice is also located in school catchment zone A, then they can send their daughter to that school; otherwise, they cannot.
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This map establishes that Sarah lives within the Zubaidah Girls Elementary School catchment area. |
Mrs. Al-Meeri, being a resourceful woman, calls the ArcView Advisor explaining her situation. She provides him with only the name of the school, Zubaidah Girls Elementary School and the street name on which she lives, Dar Al Salam. She is asked to indicate in which city she lives, Doha. Based on these seemingly independent pieces of information, the ArcView Advisor informs Mrs. Al-Meeri to expect an answer within 10 minutes!
There are several ways to find the answer to this problem, but we will consider only one. The solution will use two ArcView controls: the Query Builder button to select a feature based on some tabular attribute and the Select By Theme menu item to select other features that have a spatial relationship to the currently selected feature.
To determine the school's catchment zone we require the School theme and the School Catchment Zone theme. The Ministry of Education maintains both sets of geographic data where Schools are represented as point features and School Catchment Zones are represented as polygon features. Select the school feature by name using Query Builder and then use Select By Theme to select the catchment zone features which Completely Contain the selected school feature. This query should sound familiar, it is the point-in-polygon problem. The spatial relationship Complete Contains ensures that at most one zone will be selected.
Determining the Al-Meeri's catchment zone follows the same method as above, but this time we will be working with line features and polygon features. We will require the Street Name theme (line feature) maintained by the Roads Division, the City theme (polygon feature) maintained by the Planning Department and of course the School Catchment Zone theme (polygon feature). The reason why we need the City theme is that it may be the case that two streets have the same name, but lie in different cities! Therefore, we need to select only those streets that lie in a particular city. Select the city by name using the Query Builder. With Select By Theme, select all the streets that Intersect the selected city. Use Query Builder to select the street by name from all the selected streets. And finally, use Select By Theme again to select the catchment zone that Intersects with the selected street.
Moments later the phone rings, it's the ArcView Advisor. Mrs. Al-Meeri, in the middle of a tea majlis she is hosting, is delighted to learn that her daughter Sarah will be able to attend the Zubaidah Girls Elementary School and proudly announces the news to her guests.
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