Digitizing with an Introduction to Uncertainty and MetadataReading Assignment: Bolstad Chapter 4

Where does data come from?

• Surveying– Existing Hardcopy Maps

• Aerial photography & satellite imagery• GPS

– Field surveys, often in MS-Excel

• Direct to Internet – Twitter

Geodesy• The science of measuring the shape of the

Earth, and map projections, the transformation of coordinate locations from the Earth’s curved surface onto flat maps.

NOAANOAA

NOAA

Landsat 8

• Longest continuous satellite data set• Entire earth, twice a month, at 30 meters

GPS

• ~20 meter to <1 meter accuracy• $100 to $5000

TrimbleGarmin

Digitizing: The process of converting features on a map or image into digital vector data.

Basic Methods1. Manual digitizing (tracing!)

2. Scan digitizing

Manual Digitizing: 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

Hard Copy Digitizing– Digitizing Tablet– Puck

Map Scale: The relationship between distance on the map and distance on the ground.

Larger Scale Map Objects are larger (more detail)

Area Shown: 1 square mile

Smaller Scale Map Objects are smaller (less detail)Area Shown: 107 square miles

1/24,000 or 1:24,000 1/250,000 or 1:250,0001 unit on map equals 24,000 of same unit on the ground.

1 unit on map equals 250,000 of same unit on the ground.

Digitizing Errors•Undershoots•Overshoots

Set a fuzzy tolerance (snapping tolerance)Used to reduce undershoots and overshoots.

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 precisionThe minimum distance below which points cannot be effectively digitized as separate locations.

3. The abilities and attitudes of the person digitizing.• Attention to detail• Ability to concentrate • Steadiness of hand (too much coffee)

Scale – 1:100,000 Scale – 1:5,000

Metadata

• Data about the data• What is really critical?

– Source of the data (URL)– Contact for additional information– Date of acquisition

• Next most critical?– Collection/processing protocol

• How the data was collected and/or processed– What was the original purpose?– Approximate uncertainty

Standards Organizations

• FGDC:– Federal Geographic Data Committee

• New federal geospatial platform:– www.geoplatform.gov

• ISO:– International Standard Organizations

• Many others within specific fields of interest

Metadata Standards

• FGDC:– http://www.fgdc.gov/standards/projects/FGDC-standard

s-projects/fgdc-endorsed-standards

• ISO 19115:2003– http://www.iso.org/iso/catalogue_detail.htm?csnumber=

26020– Have to pay for it from ISO