PROPOSED STANDARD FOR DIGITAL CARTOGRAPHIC DATA QUALITY
Text adopted by National Committee for Digtial Cartographic Data Standards
1987
Later adopted nearly unchanged in Spatial Data Transfer Standard
from The American Cartographer
special issue (Vol. 15, No. 1 [January 1988])
[which was adopted in 1992 as the Spatial Data Transfer Standard]
SCOPE, PURPOSE, AND APPLICATION
The purpose of the quality report is to provide detailed information for
a user to evaluate the fitness for a particular use. This style of standard
can be characterized as "truth in labelling", rather than fixing
arbitrary numerical thresholds of quality. To implement the standard, a
producer is urged to include the most rigorous and quantitative information
available on the components of data quality described below.
MAINTENANCE
The quality standard refers to a number of other standards maintained by
related professional bodies in the surveying and mapping disciplines. This
standard should be considered to reference the currently accepted version
of these standards, as revised and promulgated by their maintenance bodies.
CONFORMANCE
Where the spatial variation in quality is known, a quality report must record
that variation.
Quality Overlays
For those components of quality displaying spatial variation, a quality
overlay system may be used. The producer of the quality report may choose
to produce a comprehensive quality overlay describing all components of
quality, or various components may be portrayed on separate overlays. When
the quality report is issued on paper, the quality overlays appear as diagrams
with text labels or thematic map depictions. In digital form, the overlays
are encoded using the standards of Part II.
COMPONENTS OF A QUALITY REPORT
Digital cartographic data shall include a quality report.
This standard describes the five sections required in the quality report:
lineage, positional accuracy, attribute accuracy, logical consistency and
completeness.
Each section of the report will contain reference to temporal information
and currency. Where the spatial variation in quality is known, a quality
report must record that variation.
The statement prescribed by the National Map Accuracy Standards (Bureau
of the Budget, 1947) does not provide a complete quality report. It is recognized
that the National Map Accuracy Standard statement may constitute the sole
quality report available for certain existing products.
Form of a Quality Report
The quality report can be issued as a paper document or encoded on computer-compatible
media in the form prescribed by Section 1.2.5.2 of this Standard. Since
the quality report will function in the assessment of fitness for use, it
shall be obtainable separately from the actual data. The digital data transmission
may contain the quality report, in whole or in part, but, as a minimum,
it must contain a reference to the quality report and how to obtain it.
Testing
In sections 2.3 and 2.4 of this Standard, there are options described for
different categories of testing. Informed assessment of fitness for use
is best served by the most rigorous types of tests. However, this standard
leaves the level of testing optional.
LINEAGE
The lineage section of a quality report shall include a description of the
source material from which the data were derived, and the methods of derivation,
including all transformations involved in producing the final digital files.
The description shall include the dates of the source material and the dates
of ancillary information used for update. The date assigned to a source
shall reflect the date that the information corresponds to the ground, however,
if this date is not known, then a date of publication can be used, if declared
as such.
Any data base created by merging information obtained from distinct sources
must be described at sufficient detail to identify the actual source for
each element in the file. In these cases, either a lineage code on each
element or a reliability overlay will be required. A reliability overlay
is a collection of points, lines and areas organized to represent quality
information for another set of map information. If a reliability overlay
is transmitted in digital form, it shall be encoded according to the standards
of Section 1 of this Standard.
The lineage section shall also include reference to the specific control
information used, whether benchmarks or triangulation stations. Control
from the National Geodetic Reference Network shall be identified according
to identifiers in that system, while other points used for control shall
be described with sufficient detail to allow recovery.
The lineage section shall describe the mathematical transformations of coordinates
used in each step from the source material to the final product. The locations
of any control points for coordinate transformations shall be given. The
methods used to make coordinate transformations must be documented. To fulfill
this standard, it is acceptable to make reference to separate documentation
for the coordinate transformation algorithm used, but the specific parameters
applied must be described for the particular case. Documentation of a transformation
algorithm must include the nature of computational steps taken to avoid
loss of digits through roundoff and must include a set of sample computations
including numerical values of coefficients to confirm equivalence of transformations.
The documentation of a transformation algorithm must be available on request
by a user obtaining digital data even if that user is not licensed to use
the particular software. .s 2 .tp 5
POSITIONAL ACCURACY
All coordinates used for the transfer of digital cartographic data must
have a known (and expressed) relationship to latitude and longitude. This
standard is implemented by the use of currently recognized standard reference
ellipsoids (for horizontal measurements) and standard geoids (for vertical
measurements). These standards are set by the Federal Geodetic Coordinating
Committee (1974). The dates of the geodetic standards and of the datum used
must be referenced.
Quality of control surveys must be reported using the procedures established
in the geodetic standard. If a separate control survey has been used, it
must be described in the standard form, even if results fall below the recognized
classification thresholds.
Descriptions of positional accuracy must consider the quality of the final
product after all transformations. The information on transformations forms
a part of the lineage section of the quality report.
Measures of positional accuracy can be obtained by one of the following
optional methods:
Deductive Estimate
(based on knowledge of errors in each production step)
Any deductive statement must include reference to complete calibration tests
and must also describe assumptions concerning error propagation. Results
from deductive estimates must be distinguished from results of other tests.
Internal Evidence
FGCC procedures will be used for tests based on repeated measurement and
redundancy such as closure of traverse or residuals from an adjustment.
Comparison to Source
When using graphic inspection of results ("check plots") the geometric
tolerances applied must be reported, and the method of registration must
also be described. Use of check plots shall be included in the lineage section.
Independent Source of Higher Accuracy
The preferred test for positional accuracy is a comparison to an independent
source of higher accuracy. The test must be conducted using the rules prescribed
in the proposed Accuracy Specifications for Large-Scale Line Maps (American
Society of Photogrammetry, 1985). The definitions of independence and higher
accuracy in the ASP standard apply. When the dates of testing and source
material differ, the report shall describe the procedures used to ensure
that the results relate to positional error, not to temporal effects. The
numerical results for precision and bias, as well as the number and location
of the test points must be reported. A statement of compliance to a particular
threshold is not adequate in itself.
This test may only be applicable to well-defined points.
The report of any test of positional accuracy shall include the date of
the test.
ATTRIBUTE ACCURACY
Accuracy assessment for measures on a continuous scale shall be performed
using procedures similar to those used for positional accuracy.
Accuracy tests for categorical attributes can be performed by one of the
following methods. All methods shall make reference to map scale in interpreting
classifications.
Deductive Estimate
Any estimate, even a guess based on experience, is permitted. The basis
for the deduction must be explained. Statements such as "good"
or "poor" should be explained in as quantitative a manner as possible.
Tests Based on Independent Point Samples
A misclassification matrix must be reported as counts of sample points crosstabulated
by the categories of the sample and of the tested material. The sampling
procedure and the location of sample points must be described.
Tests Based on Polygon Overlay
The misclassification matrix must be reported as areas. The relationship
between the two maps must be explained; as far as possible, the two sources
should be independent and one should have higher accuracy.
The report of a test of attribute accuracy shall include the date of the
test and the dates of the materials used. In the case of different dates,
the report shall describe the rates of change expected in the phenomena
classified.
LOGICAL CONSISTENCY
A report on logical consistency shall describe the fidelity of relationships
encoded in the data structure of the digital cartographic data. The report
shall detail the tests performed and the results of the tests.
Tests for permissible values can be applied to any data structure. Such
a test can detect gross blunders, but it does not ensure all aspects of
logical consistency.
A data base containing cartographic lines can be subjected to the following
general questions:
- Do lines intersect only where intended?
- Are any lines entered twice?
- Are all areas completely described?
- Are there any overshoots or undershoots?
- Are any polygons too small, or any lines too close?
Different tests can be applied to address these questions, but the quality
report shall contain a description of the tests applied or a reference to
documentation of the software used. The report shall state whether all inconsistencies
were corrected or it shall detail the remaining errors by case.
Specific Topological Tests
For exhaustive areal coverage data transmitted as chains, or derived from
chains, it is permissible to report logical consistency as "Topologically
Clean", under the condition that an automated procedure has verified
the following conditions:
The report must include the date on which the tests were applied. When corrections
and modifications occur after the test for logical consistency, the quality
report should indicate how the new information is checked for logical consistency.
COMPLETENESS
The quality report must include information about selection criteria, definitions
used and other relevant mapping rules. For example, geometric thresholds
such as minimum area or minimum width must be reported.
In encoding cartographic features, standard geocodes (such as the feature
codes described in Section 3.2 or in the FIPS codes for states, counties,
municipalities and places) shall be employed as far as possible. Deviations
from standard definitions and interpretations must be described.
The report on completeness shall describe the relationship between the objects
represented and the abstract universe of all such objects. In particular,
the report shall describe the exhaustiveness of a set of features. Exhaustiveness
concerns spatial and taxonomic (attribute) properties, both of which can
be tested. A test for spatial completeness can be obtained from topological
tests for logical consistency that respond to the questions in 2.4. Tests
for taxonomic completeness operate by comparison of a master list of geocodes
to the codes actually appearing in the file. The procedures used for testing,
and the results, shall be described in the quality report.
REFERENCES
American Society of Photogrammetry, Committee for Specifications and Standards,
1985, Accuracy Specifications for Large-Scale Line Maps, to appear in Photogrammetric Engineering
and Remote Sensing.
Bureau of the Budget, 1947, National Map Accuracy Standards: Washington
DC, GPO reprinted in M.M. Thompson, 1979, Maps for America: U.S.Geological
Survey, Reston VA, p. 104
Federal Geodetic Control Committee, 1974, Classification, Standards of Accuracy,
and General Specifications of Geodetic Control Surveys: Washington DC, GPO
1980-0-333-276 (also NOAA--S/T 81-29)
Merchant, D. 1982, Spatial Accuracy Standards for Large-Scale Line Maps:
paper presented at ACSM-ASP Fall Technical Meeting, Denver CO, reprinted
in Report 1, Columbus OH, NCDCDS
White, M.S. 1978, A Geometric Model for Error Detection and Correction:
Proc. AUTO-CARTO III, p. 439-456