Introduction to spatial data base: ArcView
3.3
April 5, 2011
ArcView is a widely used geographic information system (GIS) used to store, analyze and display spatial data. Many technical jobs require a working knowledge of GIS.
A GIS is not simply a map holder and display tool. Tables associated with maps are databases that can be queried.
You save your work as a GIS project. My projects consist of several views, several tables and layouts. A view consists of one or more themes. Themes are either raster data (images) or vector data (shapefiles). Vector data are stored as mathematical expressions.
Although a GIS can display properly prepared images (examples: satellite scan, paper topographic map that has been scanned and georeferenced - tagged with the information that links the image to coordinates), the basic ArcView computer element is a shapefile together with its associated table.
Features of a shapefile can be points (example: locations of trees with emerald ash bore infestations), lines (example: Toldeo streets) or polygons (example: Toledo Metroparks. A point is a location without real size, just like the first points you met in high school algebra. A line, or arc (the origin of the software title), connects 2 or more points. A line has length but no width (again, recall Algebra 1). A polygon is defined by a series of arcs in which the first point and the last point are identical. A polygon has area.
Download and save this file (in Explorer, right click and 'save target as') to a new folder (call it ArcView files) on your thumb drive, then unzip it and save the contents to that same folder. I will use this shapefile for the demonstration. Your assignment involves a different data set (see below).
Today you will learn how to add data to a view, classify those data and then prepare and export a slide (image) that can be pasted into Word, PowerPoint or other applications.
The ArcView button should be displayed on your desktop. If not, Start - Programs - ESRI should allow you to start the program.
You should see this dialog box (below). Do not uncheck the 'Show this window' option. Hit "OK".
You
have just created
a new project with a new View. My projects include Views, Tables and
Layouts. I like Excel for making charts and export results of
queries when charts are called for. I do not know how to write scripts for
ArcView.
When asked if you want to add data to the view, respond "Yes". Navigate to the folder with the unzipped shapefile and add that shapefile to the view. (View - Add Theme can be used to add more themes). Once you display the map, Save your project (File menu). Give your project a descriptive name.
(Click thumbnail to enlarge).
I will demonstrate the following keys:
Examine theme table
Magnify
Select
Identify
Classify
Set the View - Properties so that the project knows which coordinate system is in use (this shapefile uses latitude and longitude, degrees).
Note how changing the projection changes the shape of the map.
There are several ways to classify: let us select graduated color. The dialog box changes! Select Pop_admin - this is population of each Department (Central American equivalent of States). Click Classify.
Natural breaks looks for gaps and clusters.
Equal Area will group so as to equalize as much as possible the area of each
group.
Equal interval groups according to the number ranges, each range is the same
size.
Quantile puts (as close as possible) the same number of people (as this is a
population field) in each group.
Standard deviation does just that (not appropriate for this data set).
Note that, once you classify, you can add to and edit the Legend values and symbols. Each time you classify you lose any editing of the Legend.
Normalize: some departments are significantly larger than others. How do you analyze to get population density?
Normalize population by area. The results should be people per square km or square mile. Note how the map changes according to the classification scheme (natural breaks, equal area, etc.). A popular publication Lying with Maps discusses how information can be displayed, exaggerated or concealed.
Demonstrate selecting by query, select by mouse, convert selected to shapefile.
Demonstrate adding graphics and text to the display.
Now click the little black X in the upper right of the view (NOT the red box that closes the project).
This box is used to open additional views, add tables (more on this in 2
weeks), and layout.
Select Layouts. I will demonstrate how to prepare a figure for printing or export as image.
Each layout must have a scale and a north arrow - unless the map covers an area the size of a continent or larger. Bar scales and 'north' are distorted when we view areas so large they are best shown as part of a globe rather than a small part of this globe flattened to fit on a sheet of paper.
The right-hand button in the bottom row on the tool bar allows
you to bring a view into the layout as well as a north arrow, bar scale and
legend. The button to its left can draw lines, dots, polygons and so on.
The "T" button is for text. I will demonstrate each of these.
Recall the gold map project. Each map must have a title, north indicator,
bar scale and legend (if data on the map have been classified). Also, sign
your work!
Export (File menu) allows you to turn your layout into a .jpg. Please use this format for your assignment. Other formats generate images that require large files.
Assignment: open a new view. View menu - Add Theme. Navigate to the ESRI file (desktop's "C" drive) for some data: World - Countries. In case you cannot locate this shapefile, here is a link to data you will need, (download, upzip and store as before).
Prepare a map showing population density around the world, classify by quantiles (5 is about right) and export the .jpg image (including legend) so that you can attach it to a Blackboard Message and send it to me by Noon, Tuesday, April 12.
You might note a negative number when you attempt to classify population or population density. This does not make sense - how can a land mass have a population less than zero? Often, a database will use a preposterous value (-999) where one would expect zero or a positive integer to indicate 'no data'. How do you find and fix this before going on? The query builder button in ArcView is the magic hammer.
A simpler approach is, visit the theme table, select the population field and sort ascending. Are the values at the top of the table less than zero? Select those fields with population less than zero.
Now return to the view. Zoom to selected - recognize it? If not, zoom out until you see where in the world this special place is. Now under the 'theme' menu select 'edit theme'. Hit the 'delete' key and that selected island DISAPPEARS! No, the island is still there, Dr. Evil, but its information has simply disappeared from your database. Under the 'theme' menu, select 'stop editing' and say 'yes' to 'save edits?' Now that the negative population has been dealt with, return to your classification task.