My shortcuts to ArcView
There are no books titled ArcView for Dummies or ArcView for Idiots. For one thing, dummies and idiots cannot deal with ArcView (the application is not as intuitive as most Windows Office programs). For another, ESRI (the company that develops and markets the "Arc" family of computerized geographical information systems) is very protective of its copyrights.
ArcView is one of several geographical information systems. ArcGIS is a newer, more sophisticated GIS put out by ESRI but I have not had time to learn ArcGIS (also, ArcGIS is like Microsoft Office – lots of built-in features that make gigabyte hard drives and lots of RAM mandatory). Anyone who has ArcGIS experience should speak up when we are doing something in ArcView (or other software) that is more easily accomplished in ArcGIS.
GIS – geographical information system – note that this term is 1/3 ‘geography’ and 2/3 database (information system). ArcView is more than a handy way to make neat maps; it provides a means to develop information from spatially distributed data.
Each of my ArcView adventures is organized into one or more projects. A project consists of views, tables and layouts. There are also charts and scripts but I do not use these. ArcView charts are not as user-friendly as Excel charts (are there are even nicer chart-making applications) so I export data when it comes time to make a chart. Scripts are internal ‘programs’ that perform some complex analytical or filtering function – I have not learned how to write scripts (I wonder if there is a script that deletes duplicate points from a shapefile?).
You will do most of your work in views. A project can have many views. Each view can consist of many data layers. Each data layer is called a theme.
There are 2 kinds of themes: vector themes and image themes.
A vector theme may consist of points, lines (arcs) or polygons. Vector themes are managed as shapefiles. A shapefile consists of geographical features (points, lines or polygons) and the associated table. The table contains the attributes of the features.
A point has a location but no size, just like a point in algebra. A point theme consists of one or more points and its associated table.
An arc connects 2 points. It has a length but no width. A line theme consists of one or more lines, each consisting of one or more arcs, and the theme table. ArcView keeps track of where a line theme begins and where it ends.
A polygon consists of a series of arcs (line segments) where the line begins and ends on the same point.
ArcView can perform analytical operations on shapefiles because the mathematical information is available (but not displayed in the tables).
An image is a matrix (rectangular array of numbers) which the computer interprets as colors (one value represents red, another green, another blue, etc.). Most image files I use are geotiff format or SID format. In order to use SID format images, you need to activate the Mr.SID image support extension (File menu). SID images use a special compression method that I do not know how to perform. SID images require less disk space that geotiffs covering the same geographical area at the same resolution. For example, it takes 6 or 7 CD-ROMs to store the scanned topographic maps of Honduras, but a single CD-ROM holds these maps stored in SID format.
An image must be registered in order to pop up in ArcView in the correct geographical location. Usually, an image theme has the image plus a short text tile with the same name (different, very specific extension) in the same folder. When you download the image file you must also download the worldfile, the partner file with the information specifying location and size of the image.
Images are displayed as rectangular elements called rasters. Elements on your monitor are called pixels. If we get too close to an image, the image deteriorates into blocks of varying colors. High-resolution images are nice but take up a lot of storage space.
Units, projections and datum choices
A common problem in GIS occurs when data from different sources (or even from the same source) employ different units (degrees, meters, kilometers, feet, miles, cubits).
Two common coordinate systems are geographic and UTM (universal transverse Mercator, sometimes called military grid). Degrees are useful for projects covering hundreds of kilometers, where the curvature of the earth becomes a factor. Smaller areas approximate flat maps more closely and the Cartesian XY axes (in feet, meters or kilometers) makes life easier for map users. Artillery spotters tell the fire officers the grid location of the target, and the fire officers calculate distances and direction based on the relative positions of guns and targets using a little bit of plane geometry and trigonometry.
What is a map? A map is a graphical representation of spatial reality. No flat map is perfectly accurate. In transforming features from an oblate spheroid (the earth is not even perfectly round!) to a flat piece of paper requires distortion. What becomes distorted and what is most closely preserved depends on the projection selected. We are not going into the many projects available and their many advantages and disadvantages (you cannot preserve relative directions, distances and areas on the same map – something is lost no matter how the map is drawn) – Geography & Planning can teach you all about map projections.
Regarding datum: geographic coordinates are measured in degrees north or south of the equator, and east or west of the Prime Meridian, a reference ‘line’ running from the south pole to the north pole through the British Royal Observatory in Greenwich, England. UTM coordinates begin with zone, each zone 6 degrees wide with a center meridian midway between meridians with values which, when divided by 6, yield an integer. To the central meridian value of X = 0, the constant 500 km is routinely added so that all values are positive. The equator is Y = 0 for values in the northern hemisphere.
There are 2 frequently employed UTM systems in use: NAD27 and NAD84. One lies about 200 meters north of the other at our latitude. Another UTM-like system you are likely to encounter is a state plane coordinate system, which might use feet as units as well as X and Y axes centrally located so that areas and distances in that state can be specified with the least error possible in transforming from the curved earth to the flat map. Just as states employ different coordinate systems centered on their geographical locations, many parts of the world have their own datum.
Each view must consist of themes sharing the same coordinate system – units and datum. There is an ArcView conversion utility that can convert shapefiles from one coordinate system to another – but the user has to know both the present coordinate system and the desired coordinate system. Metadata is a file that should accompany every GIS theme. Metadata are data about the data – information on coordinates, resolution (image themes), units, who did the work and how the data were developed (surveying? GPS? Scanned from paper maps?).
I like to use the Theme – Properties to keep track of this sort of information. The same box is there you specify the units you want to use in measuring distances on the view. You may, for example, use a view with meters as units but measure distances in miles.
Opening a new project
Adding themes to the project
Examining a theme table
Classifying vector data
Navigating the view
Setting up a layout
Exporting a layout
Saving your project
Note: saving your project does not save along with it all of the themes. What you save is instructions to ArcView where the shapefiles and images are stored. Taking your project file to a new computer will not enable you to continue your work unless you have with you all of the files accessed.
For next week: prepare 2 layouts: one demonstrating classification of some feature using color and the other classifying a point feature by size. If you have no data of your own, use shapefiles in the ArcView folder or from the CD distributed in class. Note that you cannot use both color and size simultaneously for the same feature (you cannot use color to classify earthquakes by depth while using size to classify the same earthquakes according to magnitude).
Export the layouts as .jpg files and send them to me attached to email messages by next Thursday afternoon. Please include your name on each layout. Don’t forget to save your projects!
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