1. Introduction to Geographic Information Systems
GLY 560: GIS for Earth Scientists
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2. What is a Geographic Information System (GIS)?
A broadly accepted definition of GIS is the one provided by the National Center of Geographic Information and Analysis (NCGIA):
A GIS is a system of hardware, software and procedures to facilitate the management, manipulation, analysis, modeling, representation and display of georeferenced data to solve complex problems regarding planning and management of resources (NCGIA, 1990)
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3. What is a Geographic Information System (GIS)?
Or simply:
An information system designed for spatially referenced data.
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4. What is a GIS? Computer Brain Power Soft- ware Spatial Data
A GIS can be thought of as in integration of a conceptual database design (Brain Power), spatial data sources, GIS software, and a computer. The system allows more spatial data to be organized than could be accomplished in your head, and in some cases more than could be accomplished with maps. GIS allows data to be stored and then retrieved “on-the-fly” so that maps may be generated very quickly for specific purposes.
In it’s essence, a GIS is just a way of generating maps very quickly and efficiently. We are still limited to
Spatial Data
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5. Limitations of GIS
In spite of the considerable power of GIS systems, it is still really just a map-making machine. We are still converting essentially three dimensional data to two dimensions.
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6. Limitations of GIS
GIS does make “psuedo” 3-D (+z) and 4-D (+z + t) possible by linking elevation and time-series data to geographic position.
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7. How GIS Works Information is stored in layers.
Similar data types or “themes” are stored in each layer.
Data queries are given in terms of what layers are desired
Data are then displayed by overlaying all data requested on a single map.
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8. Geospatial Data
Geospatial data have both spatial and thematic components:
Spatial component: The observations have two aspects in its localization:
absolute localization based in a coordinates system
topological relationship referred to other observations.
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9. Geospatial Data
Geospatial data have both spatial and thematic components.
Thematic component: The variables or attributes can be studied considering the thematic aspect (statistics), the locational aspect (spatial analysis) or both.
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10. Types of Spatial Data
Summary of GIS Data Types
Raster Data: data represented as a regular grid. (e.g. digital elevation model or DEM, images)
Vector Data: data represented as points, lines, or polygons. (e.g. roads, streams, boundaries)
Non-Topological Vector Data: no relationship between vector data objects (handled primarily by ArcView)
Topological Vector Data: explicit spatial relationships among features (handled primarily by ArcInfo)
Higher-Order Data: explicit grouping of topologic data (TINS based on elevation points, regions that are related)
After Chang, 2002, Fig. 1.1
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11. Vector vs Raster Representation of Data
Vector Representation
The top graphic above illustrates how geographic entities might be represented with vector data. The small squares are nodes—point locations specified with latitude and longitude coordinates. Line segments connect nodes to form line features. In this case, the line feature colored red represents the highway. Series of line segments that begin and end at the same node form polygon features. In this case, two polygons (filled with blue) represent the reservoir.
The bottom graphic illustrates a raster representation of the same reservoir and highway. The area covered by the aerial photograph has been divided into a grid. Every grid cell that overlaps one of the two selected entities is encoded with an attribute that associates it with the entity it represents. Raster data consists of a list of numbers, one number for each grid cell, each number representing an entity. For example, grid cells that represent the highway might be represented with the number 1 and grid cells representing the reservoir might be coded with the number 2.
Raster Representation
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12. Raster Data
Data are divided into rows and columns
Location is implicit in structure (i.e. size of each cell).
Each cell may be given values or “attributes”.
Data are divided into rows and columns, which form a regular grid structure. Each cell must be rectangular in shape, but not necessarily square. Each cell contains location as well as an attribute value.
The spatial location of each cell is implicitly contained within the ordering of the matrix, unlike a vector structure which stores topology explicitly.
Cells may be grouped together according to attributes, but cannot be stored as a group.
Figure after:
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13. Non-Topological Vector Data
Non-topological vector data can be points, lines, or polygons. ESRI calls non-topology-based maps “shapefiles”.
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14. Topological Vector Data
Topological Vector Data
Topological features “know” where they are with respect to other features. There is a left and right side to an arc.
Topological vector data can be label points, nodes, arcs, or polygons. ESRI calls topology-based maps “coverages”.
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Figure 8. ARC / NODE structure or POLYVRT

15. Topologic vs. Non-Topologic Data Models
Display quickly
Simple and compact data storage
Difficult to find and fix errors
Spatial queries more difficult
Method of choice when need a “simple map”
Topologic
Display more slowly
More complicated and less compact data storage
Can “clean up” maps easily.
Facilitates advance spatial queries.
Can easily develop measurements along routes
Method of choice when need “smart map”
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16. Attribute Data
Features in a GIS usually have associated data called “attributes”.
Each feature may have a number of attributes, each attribute may have a number of features.
Attribute table for counties in Pennsylvania.
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17. Georelational Data Model
Attributes are stored in a relational database and linked to features. This is called a “georelational data model”
Attribute table for counties in Pennsylvania.
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18. Map Projections and Geodesy
With a GIS you still have to project 3D globe on a 2D surface
Must choose the appropriate projection
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19. Projections and Datums
You must decide what map projection to use
You must decide what horizontal datum to use.
GIS systems allow you to convert among projections and datums efficiently.
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20. Geologic Maps Legend Scale Coordinates North Arrow 4/26/2017
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21. GIS Generated Maps
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22. Data Sharing
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23. Summary GIS good for 2D static spatial data
Can be vector or raster data
Topologic or non-topologic data
Enables reprojection
Data can shared over internet
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