Introduction to Digitizing

 

The process of converting features on a map or image into digital vector data is called digitizing. There are two basic methods of digitizing:

• Manual digitizing (tracing!)
• Scan digitizing (automated)

Scan digitizing works for some data sets but we'll be using manual digitizing as it always works.

 

Manual Digitizing

Manual digitizing is human guided coordinate capture from a map or image source (tracing).

1. Heads-up Digitizing (on-screen)

• Manually digitizing on a computer screen using a digital map or image as a backdrop
• Raster -> Vector

2. Hardcopy Digitizing

• Manually digitizing on a digitizing tablet using a hardcopy map or image.
• Hardcopy -> Vector

If you need to digitize a large amount of information, you may want to use hardcopy digitizing as it can be more accurate but it requires a tablet to be efficient. We'll be doing "Heads-up Digitizing".

Map Scale

Map scale is the relationship between distance on the map and distance on the ground. Knowing the scale you are digitizing at is critical to maintaining accuracy of the data you are creating.

• Larger Scale Map: Objects are larger (more detail), as if you're zoomed in
• 1/24,000 or 1:24,000 - 1 unit on map equals 24,000 of same until on the ground
• Area shown below: 1 square miles

• Smaller Scale Map: Objects are smaller (less detail), as if you're zoomed out
• 1/250,000 or 1:250,000 - 1 unit on map equals 250,000 of same unit on the ground
• Area shown below: 107 square miles

The scale of your map is not fixed in a GIS. It can be adjusted in ArcMap using the map scale drop down tool:

A larger scale of the same map:

Note: Many people confuse the terms "small" and "large" scale. Remember that features appear "small" on a "small scale" map and appear "large" on a "large scale" map.

Digitizing Errors

Positional errors are inevitable when data are manually digitized.

• Undershoots
• Overshoots

Setting a fuzzy tolerance (snapping tolerance) is used to reduce undershoots and overshoots. The snapping tolerance is the minimum tolerated distance between nodes, lines and/or vertices.

Characteristics of manual digitization that may negatively affect positional quality of spatial data:

1. Map Scale (not a problem for heads-up). Digitizing from small scale maps will introduce larger positional errors.
2. Device precision. The minimum distance below which points cannot be effectively digitized as separate locations.
3. The abilities and attitudes of the person digitizing.
   1. Attention to detail
   2. Ability to concentrate
   3. Steadiness of hand (too much coffee)

Below is an example of digitizing affected by map scale:

Grizzly Lake Trinity Alps Wilderness

Test Your Knowledge

 

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