Basics of Digitizing

Digitizing is the process of converting analog information into a digital representation. In regards to spatial information one application of this is the process of creating a vector digital database by creating point, line and polygon objects. Scanning a map can also be considered digitizing (turning colors shades on the map into digital values), but for this class when we refer to digitizing this for the most part refers to creating vector datasets.

Some basic issues related to digitizing:

1) Digitizing point features is simply the process of creating a single point feature with an x,y coordinate

2) Digitizing lines involves creating a line feature that consists of a node (start and end) and a series of vertices which indicate a change of direction along that line. Straight features require fewer vertices, curved/complex features require more vertices.

3) Digitizing Polygons involves creating a set of connected lines. Editing tools simplify the process of closing polygon features

4) It is often the case that you want to attach or snap new features to existing features in a dataset or existing features in another dataset. This assists in reducing overlaps and intersections of features and creating topology. This can be done by setting the snap environment. From the Editor toolbar, choose Snapping to turn snapping on/off for different features. From Editor toolbar choose Options to set snapping distance.

5) Digitizing involves creating features from a specific scale. Small scale maps (i.e. maps of large spatial areas - given a fixed map size) generalize features, large scale maps (i.e. maps of small spatial areas - given a fixed map size) represent features more accurately with less generalization. A key is that the digitized dataset you create is only as good as your source data.

Here are some general rules/guidelines/tips:

1) In ArcGIS you can edit shapefiles and geodatabases. If your installation has the ArcInfo license of ArcGIS then you can also edit coverages (the STC's have this functionality). Since geodatabases have several advantages over other vector formats, we'll focus on these in class.

2) A Geodatabase can be what is called a Personal Geodatabase or a Server Geodatabase. Basically, a Server Geodatabase can be simultaneously accessed by multiple users while a Personal Geodatabase is locked when a user starts to edit or modify it.

3) Features for a Geodatabase are contained in Feature Classes. A Geodatabase can contain multiple Feature Class types (point, line, polygon) but only one feature class type can exist within a specific Feature Class.

4) A Feature Dataset is a sub-unit of a Geodatabase. If you have a series of datasets all for the same study area and in the same spatial reference system, you can put them in the same Feature Dataset to facilitate the definition of their spatial domain. In other words, you can have a Geodatabase in which is a Feature Dataset, and several Feature Classes within that Feature Dataset. Or you can just put a Feature Class in the Geodatabase without a Feature Dataset hierarchy.

5) One of the main advantages of the Geodatabase structure is the ability to create topological rules that define spatial relationships between features within a Feature Class or between features within different Feature Classes. These topological relationships are created within a Feature Dataset for Feature Classes within that Feature Dataset.

6) Another advantage of Geodatabases are attribute domains. Attribute Domains are used to restrict the valid values for a particular attribute. Valid values can be restricted to a specific set of nominal categories (Oak, Hickory, Maple) or data ranges (valid values must fall between 10-50).

In class we will go through the following basic steps to creating and digitizing features:

  1. Create a new Personal Geodatabase
  2. Optional step: create a Feature Dataset within the Personal Geodatabase
  3. Create a Feature Class within either the Personal Geodatabase or Feature Dataset
  4. Define the Spatial Reference Information for the Feature Class
    1. If the Feature Class resides within a Feature Dataset, it will adopt the spatial reference information of that Feature Dataset
    2. You can import the spatial reference information from another Feature Class to simplify the process of defining this for your current Feature Class
    3. If your Feature Class does not exist within a Feature Dataset, a critical component to set is the X/Y Domain - this is a definition of a bounding box in coordinate space within which your features reside. If you set this incorrectly then you will not be able to add features to your Feature Class where you want and you will have to recreate your Feature Class.
  5. Optional step: create topology rules within the Feature Dataset
  6. Import, Add or edit features to the Feature Class (Import: ArcCatalog, Create/Edit: ArcMap/Editor Toolbar)
  7. Optional step: check for topological errors and correct