Digitizing with an Introduction to Uncertainty and Metadata Reading Assignment: Bolstad Chapter 4.
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Transcript of Digitizing with an Introduction to Uncertainty and Metadata Reading Assignment: Bolstad Chapter 4.
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