Attribute tables can be intimidating until you learn how to read them. To get a feel for how they work, load a small vector layer like roads, buildings, or administrative boundaries. Then open the attribute table before doing anything else to change the style of the map. Click on a single row and see which feature gets highlighted on the map canvas. Select a feature on the map and try to find the matching row in the table. These two actions demonstrate a simple concept: Each row typically represents a geographic feature. Each column contains a particular piece of information for the feature.
The columns are fields. In a roads layer, there might be fields for road name, road surface type, road classification, or length. In a polygon layer, there might be fields for district names, area values, or population counts. The values in a row tell us something about the selected feature. Try asking two questions while looking at an attribute table. What does a row represent? What does a field represent?
Geographic information system software interprets data differently depending on whether it is text, integers, decimals, or dates. A field that stores population counts as numbers can be easily used in a query or classification. But if population counts are stored as text, they will sort improperly (e.g., 100 would come before 20). Before creating a query, calculation, or graduated symbol map, check the field type in the layer properties or field information dialog box.
It can be difficult to read an attribute table when there are many fields. Don’t scroll aimlessly. Pick a single feature and find its row. Look only at the fields you need. You can hide fields you don’t need, widen useful fields, or sort a field to see if the values make sense. For instance, sort by area (largest to smallest) to quickly see if the values appear to be reasonable.
It is easy to confuse filters and selections. Filters control which records and features are available in the layer, whereas selections highlight records that meet a criteria but do not affect other records. Say your roads table has a field named “surface”. You can select all the records where the value is “gravel” to quickly view them on the map. You can also filter the table to only show gravel roads. Be sure to spell things correctly. Double-check the use of capitals, spaces, operators, and field names.
Sometimes the attributes table contains empty values, duplicate entries, inconsistent names, or values in strange units. Never trust a table just because it looks complete. Examine multiple records with known features, look at minimums and maximums for strange values, and check any documentation. When the selected row corresponds to the selected feature, the field type matches the expected data, and the field contents make sense in context, the attribute table stops being an abstract set of values and becomes a window into the geographic information the map contains.
