GEO 465/565
Data Quality

• Data Quality
• Introduction
• Introduction (cont.)
  • All spatial data are inaccurate to some degree, BUT data are generally represented in the computer to high precision
• Accuracy
  • the closeness of results, computations, or estimates to TRUE values (or values accepted to be true)since spatial data are usually a generalization of the real world, it is often difficult to identify a TRUE value
• Accuracy (cont.)
  • accuracy of the database may have little relationship to the accuracy of the products computed from the database
• Precision
• Accuracy vs. Precision Graphic
• Important Questions
  • since all spatial data are of limited accuracy, the important questions are:how to measure accuracy
    how to track the way errors are propagated through GIS operation
    show to ensure that users dont ascribe greater accuracy to data than they deserve
• Components of Data Quality
  • Alternative definitions. . . .conformance to expectations: fulfilling arbitrary thresholds
    following established procedures, as with geodetic standards
    fitness for use: truth in labelling (distinct roles of producer and consumer)
• Components (cont.)
  • 1982-1988: Members of the American Congress on Surveying and Mapping met to produce a draft standard
    Later became the Spatial Data Transfer Standard
• Components (cont.)
  • standard model to be used for describing digital data accuracy
    similar standards being adopted in other countries
• Components (cont.)
  • National Committee for Digital Cartographic Data Standards (NCDCDS) identifies several components of data quality:
    positional accuracy
    attribute accuracy
    logical consistency
    completeness
    lineage
• Positional Accuracy
  • defined as the closeness of locational information (usually coordinates) to the true position
    maps are accurate to roughly one line width or 0.5 mm
    equivalent to 12 m on 1:24,000 or 125 m on 1:250,000 maps
• Positional Accuracy (cont.)
  • within a database a typical UTM coordinate pair might be:
    Easting 579124.349 m, Northing 5194732.247 m
• Testing Positional Accuracy
• Testing Accuracy (cont.)
  • Compute accuracy from knowledge of the errors introduced by different sources
    e.g., 1 mm in source document
    0.5 mm in map registration for digitizing
    0.2 mm in digitizing
• Attribute Accuracy
• Attribute Accuracy (cont.)
• Logical Consistency
• Completeness
  • concerns the degree to which the data exhaust the universe of possible items
    e.g., are all possible object included within the database?
    affected by rules of selection
    • by generalization
      • by scale
• Lineage
• ERROR in Database Creation
  • error is introduced at almost every step of database creation
    What are these steps, and what kinds of error are introduced??
    POSITIONAL MEASUREMENT ERROR
  • GPS is a powerful way of augmenting this network
• GEOID
  • Geodetic control points correlate lat/long, height, scale and orientation throughout the U.S. - based on the geoid
    Geoid is the shape that would approximate by an undisturbed mean sea level - surface where gravity everywhere is perpendicular
• Geoid Map
  • Geoid heights range from lows in the Atlantic to highs in the Rockies
• Positional Measurement Error
  • most positional data on land are derived from air photoshere accuracy depends on the establishment of good control pointsdata from remote sensing is more difficult to position accurately because of the size & quantity of each pixel
• Digitizing Errors
• Digitizing Errors
  • 2 types of errors normally occur in stream-mode digitizing
    physiological - hitting the button twice or involuntary muscle spasms that tend to produce spikes, switchbacks, or loops
• Digitizing Errors
• Attribute Errors
• Compilation Errors
• Processing Errors
  • mathematical errors
    accuracy lost due to low precision computations
    rasterization of vector data
    misuse of logic
    generalization/smoothing and related problems of interpretation
• Data Quality Reports
  • Reports issued under U.S. National Standards. . . .
• Questions?
• The End!

Data Quality & Accuracy

Introduction

Introduction (cont.)

Accuracy

e.g., in measuring the accuracy of a contour, we compare the contour as drawn on the source map, since the contour does not exist as a real line on the surface of the earth

Accuracy (cont.)

Precision

Accuracy vs. Precision Graphic

Important Questions

Components of Data Quality

Components (cont.)

National Committee for Digital Cartographic Data Standards (NCDCDS) was recently developed by a coordinated national effort in the U.S.

1982-1988: Members of the American Congress on Surveying and Mapping met to produce a draft standard
Later became the Spatial Data Transfer Standard

Components (cont.)

Components (cont.)

Positional Accuracy

Positional Accuracy (cont.)

If the database was digitized from a 1:24,000 map sheet, the last four digits in each coordinate (units, tenths, hundredths, thousandths) would be questionable

Testing Positional Accuracy

Testing Accuracy (cont.)

if sources combine independently, we can get an estimate of overall accuracy by summing the squares of each component and taking the square root of the sum

Attribute Accuracy

Attribute Accuracy (cont.)

Logical Consistency

Completeness

Lineage

Additional stuff not covered in video lecture:

ERROR in Database Creation

the most accurate basis of absolute positional data is the geodetic control network = a series of points whose positions are known with high precision

GPS is a powerful way of augmenting this network

GEOID

Geoid is the shape that would approximate by an undisturbed mean sea level - surface where gravity everywhere is perpendicular

must correct for the deflections of the vertical so that measurements of distance on the earth's surface will be consistent with those determined by astronomic observations

Geoid Map

Positional Measurement Error

some positional data come from text descriptions, e.g., old surveys tied in to marks on trees or boundary following a watershed or midline of a river

Digitizing Errors

Digitizing Errors

physiological - hitting the button twice or involuntary muscle spasms that tend to produce spikes, switchbacks, or loops

Digitizing Errors Cont.

* may also involve misinterpretation or too much generalization
* not easy to remove these automatically
* in spite of physiological and psychomotor errors, digitizing itself is not a major source of positional error
* also errors in registration & control points, as well as shrinkage or shredding of paper

Attribute Errors

Compilation Errors

Processing Errors

Data Quality Reports

current standards being developed by the NCDCDS (National Committee for Digital Cartographic Data Standards) which sets standards for data quality documentation as well as components of data quality

Questions?

The End!

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