Tables are used to store descriptive information. In ArcGIS they are usually linked to a vector layer or appear individually. The alphanumeric information forming a table can be stored as an integer, decimal number, text or date. The supported table formats are diverse: Geodatabases, databases, INFO, dBASE, text files, Microsoft Excel, Access, SQL, etc.
7.1. GIS Book: Working with tables
Each shapefile contains an associated table in DBF format. To view the table of a vector file in ArcMap right click on a layer and select Open Attribute Table.
A table is structured in rows and columns. Each row can be considered as an object containing values in different fields. A column is restricted to a single data type and is considered as a field. You can add as many fields as required to a table.
Once the table has been opened, a new field can be created by going to Table Options () and selecting Add Field (note that no edition is active, if it is active, click on “Stop Editing“). Then a dialog box appears where you must specify the new field (see Figure 26). First you must name the new field, taking into account that you cannot include spaces or special characters and are limited to a length of eight characters in shapefiles and dfb tables. Then, specify the field type for which there are several options clarifying which data are allowed within this field (numerical: short integer, long integer, float or double; text: text; date: date). Finally, you specify the properties of this field (“Field Properties“), where the accuracy (numbers) and/or space (text) is limited.
To choose the type of new field, consider the information provided by (Neer, 2005):
- Short Integer, are numbers integers between -32768 y 32767.
- Long Integer, are integer numbers between -2147483648 and 2147483647.
- Float, are fractional numbers between -3.4E38 and 1.2E38.
- Double, are fractional numbers between -2.2E308 and 1.8E308.
- Text, is a text string limited to 255 characters in length.
- Date, are data stored in Universal Time Coordinates (UTC).
To enter information within each field, you need to be in Edit Mode (Editor > Start Editing). The attribute table (“Open Attribute Table“) opens and values can be entered directly in cell.
To add the same information/calculation to one or more rows, right-click on the required field (e. g. Type) and select the “Field Calculator” tool. A new dialog box opens where by default VB Script from the “Parser” section is enabled (if necessary, switch to Python). In the “Fields” section you can find all the fields of the current table, and in the “Type” section you can see the type of data (Figure 27). To enter the information, you must type the numbers directly, enclosing the text in quotation marks (” “; for example, randomly select one or more buildings and call them “School”; you can repeat the process for “Church”, “Hospital” or “Housing”) in the window at the bottom.
Practice: Add new fields to shapefiles and configure their name, type and properties as shown in Table 2.
Fill in the table values based on the following information:
- hydrant_points, in the Name field, call the first point “Main” and the second “Secondary”.
- , in the Name field, call the first and fourth points “Secure sites” and the second and third point “Meeting zone”.
- , in the Type field, name the entities that extend along the coordinate X: 691723 Y: 9532552 as “Main way“, name the segment that extends along coordinate X: 691394 Y: 9532401 as “Unpaved way”, name as “Path” the segment that extends along the coordinate X: 691570 Y: 9532693, and name as “Street” all remaining segments in the urban area.
- water_network, in the Name field call it “Malacatos River“.
- plant_cover, based on Figure 4 name the polygons in the Type field as “Grassland“, “Crops” or “Forest“, as appropriate.
- lake, in the Name field, name it “Santa Anilla Lake“.
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