Surveying GIS: What is a geodatabase?
Enter ArcGIS and the geodatabase. This evolutionary step in the “quest for the Grail” of seamless GIS/CAD interaction replaces the terms “coverages” and “themes” with “layers.”
“A geodatabase is a store of geographic data implemented with the relational database of your choice,” as defined by its developer, ESRI. The geodatabase is comprised of subsets of various other data elements including:
A Feature dataset, which contains spatially related feature classes together with the network objects that bind them. Feature classes share a spatial reference within a feature dataset.
A Feature class, which is a table with a shape field containing point, line or polygon geometries for geographic features.
A Topology, which is a set of integrity rules.
A Geometric network to address the connectivity issues of line and point features.
A Survey Dataset, which includes measurements, points, coordinates and calculations.
Raster datasets, which have individual images or raster tables.
And of course, the data tables.
What makes the geodatabase different in concept is that it serves as a single “address” or container for many related GIS layers. Rather than having a road shape file or coverage and a parcel shape file or coverage, the user has a single geodatabase containing a road layer and a parcel layer for a specified area.
ArcGIS has three desktop components: ArcCatalog, ArcMap and ArcToolbox. In ArcCatalog the title is accurately descriptive of the function. This application resembles Windows Explorer in appearance. But it contains all the necessary functionality to view and manage GIS datasets.
ArcToolbox is the application used for what is called “geoprocessing.” Geoprocessing is basically an umbrella term that means editing, changing, moving, copying or otherwise altering the data in the set. In ArcView 8.x, ArcToolbox has all the basic geoprocessing tools. The ArcGIS version of ArcToolbox also includes ArcINFO Workstation.
And ArcGIS (8.x) architecture differs from both ArcINFO and ArcView 3.x by incorporating VBA (Visual Basic for Applications) as a built-in development environment. ArcINFO used AML (Arc Macro Language), an ESRI-created programming language to create applications. ArcView 3.x had Avenue, a built-in object-oriented programming language. But both of these tools were proprietary in nature. And as such, the pool of highly skilled available programmers was limited. VBA is a Microsoft product and thus mainstream by default.
Layer PropertiesLet’s take a quick tour of the tabs in the “Layer Properties” dialog. The user activates the Layer Properties dialog by right clicking on it in the Data Frame and selecting it, or simply double clicking on the layer in the Data Frame.
One tab is “General,” which serves as a “title sheet” and sets the base layer scale. There is “Labels,” which we looked at last time. Then there is “Drawing Layers.” Drawing Layers displays all the layers in the data frame by name. In the example (see figure on page 62) it preserved the names of the layers from the original AutoCAD drawing. The check boxes and buttons allow the user to turn the individual drawing layers on and off. “Source” simply identifies the database much the same as “Theme Properties” in ArcView 3.x.
“Selection” sets the property symbology for “selected” features. Just as in ArcView 3.x the selected records and features are displayed by the symbols and colors selected with this tab. “Display” sets the thresholds controlling the scales of displayed objects. “Symbology” to a great degree creates the appearance of your map. In this dialog the user sets the type, color, shape and value of the objects in the Map View.
“Definition Query” is very similar to the query builder in ArcView 3.x. It allows the user to write expressions that produce a variety of reports and results.
“Joins and Relates” refer to actions associated with the tables. Microsoft Access users will instantly recognize the terms. This tab displays any existing “Joins and Relates” and allows the user to select and add new ones. “Joins” allows the user to merge data from external tables using a common field. “Relates” links a common field from an external table without merging.
The “Transformations” tab works more like some CAD programs than the Projection Utility Wizard in ArcView 3.x. This dialog provides three options to tranform data: the World File (.tfw), Coordinates (two points in each system) and Rotate, Scale and Translate. More complex re-projections can be performed with ArcToolbox. (Note: ArcMap assumes a *.prj (projection) file exists for the dataset.)
The Help Files in ArcGIS are not as mature as ArcINFO or ArcView 3.x. One hopes that over time the Arc 8.x suite will match the utility of earlier products. But the online help available at http://support.esri.com/ is very often useful to help handle problems.
For users with previous experience there is a course titled “Migrating From ArcView 3.x to ArcView 8.x available at the ESRI Virtual Campus at http://campus.esri.com/. The course is free and does a good job of helping the user get familiar with the changes. ArcView 3.x users that do “upgrade” to ArcGIS can import existing project (*.apr) files into ArcMap.
This little button (!)* is referred to as the “Zen of GIS.” Who says Jack Dangermond doesn’t have a sense of humor?
In this episode we have toured one aspect of ArcMap. ArcCatalog and the formidable ArcTools will be explored in the next installment (July).
*Actually part of a VBA programming customization exercise in the tutorial.