Nova Scotia's
    Geographic Information
    Standards

4.0 Introduction:

Data Quality and Accuracy issues address the heart of the data consumers concern over, and confidence in, the data they use for their particular application. Data quality standards refer to the group of standards which will enable the data consumer to assess the suitability of geographic information databases for use in specific applications. Data accuracy standards meanwhile refer to the degree to which an estimated / measured value approaches the actual / true value.

This chapter will address standards related to the documentation of quality and accuracy specifications as they primarily pertain to digital data. Selected issues are not being addressed by the Data Quality and Accuracy Task Group. Rather they are being handled by other standards task groups and/or related interest groups (e.g. Data Pricing and Distribution Task Group, under the direction of the Nova Scotia Land Use Committee). Once complete, their results will be looked at in light of the standards initiative and where deemed appropriate, will be incorporated into this chapter.

Other activities aside, the Data Quality and Accuracy Task Group was directed to address the following issues: Accuracy, Completeness, Currency, Lineage, and Structuring (sometimes referred to as topological structuring or Logical Consistency).

4.1 Topic:
Accuracies [Endnote 1] and Resolutions of Nova Scotia's Base Map Series

4.1.1 Purpose:

4.1.2 Background:

The Best of a Worst Case

When attempting to understand the accuracy of any data set, the consumer must be aware that the data set may not be created from a single source. As a result of compiling a variety of sources, the accuracy of the compilation is only as good as the worst source. This is particularly true in the digital world where an entire data set is presumed to be consistent throughout. Far too often data consumers fail to ask the question "Where does this data come from?", "How was it compiled?"

In this section standards related to a data set's accuracy and resolution will be discussed. These standards will for the most part be directed at segments of the primary databases. In particular, base maps of various scales making up the primary databases will be highlighted. The primary databases' accuracy and resolution values will be documented along with those of the databases provided as part of the national topographic series.

Accuracy - Relative vs Absolute

When attempting to document a data set's accuracy, in most instances, the individual is referring to that data set's positional accuracy. For the purpose of this discussion the terms accuracy and positional accuracy are synonymous.

Accuracy can be referred to as being either relative or absolute. Data consumers must be aware of the differences in the two types of positional accuracy. Relative accuracy refers to the position of an object in relation to its actual position. For example, to refer to a map's horizontal accuracy as 2.5 meters can suggest that the feature portrayed on that map is assumed to be within 2.5 meters of their true position on the ground. Absolute accuracy meanwhile refers to an objects position in relation to a control survey network.

Resolution

As with accuracy, resolution carries with it many meanings. It may be defined as:
      Measure of the ability of a system to distinguish detail under certain specific
      conditions. The measure of this ability is normally expressed in lines per
      millimetre, meters per pixel, dots per inch, etc.
      [ see Standards Glossary - Appendix A ]

It can also be defined as:
      a minimum distance between two objects that can be distinguished by a
      sensor. Most often a synonym for spatial resolution but also applies to
      spectral and temporal aspects of remote sensing imaging systems
      [GIS World Sourcebook 1995]

Accuracy vs Resolution

What is a data set's accuracy and resolution? To further complicate the accuracy issue, two terms Accuracy and Resolution are often interchanged. All too often an individual will refer to a data set's resolution when in fact they really are interested in its accuracy. While these terms can indeed mean one and the same thing, such a practice is not encouraged and in the context of the standards process, the two terms are very different.

Accuracy has been defined above as being of two types relative and absolute. In a more general sense accuracy has been defined as the degree to which an estimated / measured value approaches the actual / true value. With regards to the various map series listed in Tables 4.1-4.3, it is the accuracy values which most data consumers are interested.

The resolution values presented in Tables 4.1-4.3 refer to spatial resolution and are in reference to factors that affect data collection at the photogrammetric stage.

Accuracy and resolution values therefore differ as a result of the various processes a data set goes through prior to becoming distributable. Between compilation and distribution, there are a number of additional phases of data preparation which determine the data set's accuracy. Establishment of photo control, bridging, block adjustments, model adjustments, etc , all introduce data errors. These errors tend to be compounded and the result is expressed as a data set's accuracy. If one were to start and stop data set compilation with the photograph and only reference features with the photogrammetric instruments, the horizontal and vertical resolution and accuracy would be one and the same.

Horizontal and Vertical Measures

Why must accuracy and resolution be defined in terms of horizontal and vertical components? The horizontal components of a data set refer to the X and Y values while the vertical components refer to the Z values assigned to data. Because the mapping environment requires projecting a 3 dimensional surface on a 2 dimensional plane, area measurements are subject to a series of displacement and distortion errors and height measurements are prone to parallax errors. By measuring distances in the X and Y directions we are able to calculate area and distance in a two dimensional plane, measuring distance in the Z direction allows us to calculate height, volume, etc. When referring to the accuracy of a data set both the horizontal and vertical accuracies must be taken into consideration.

4.1.3 Standard: Horizontal / Vertical Accuracy / Resolution for Nova Scotia Topographic Database Products

To adopt the existing horizontal and vertical accuracies and resolutions for map scales of 1:1000, 1:2000, 1:5000, 1:10 000 (as depicted in Table 4.1).

4.1.4 Standard: Horizontal / Vertical Accuracy / Resolution for Associated Products

To adopt the existing horizontal and vertical accuracies and resolutions for DEM and Property based data and map scales of 1:50 000, 1:250 000 and 1:500 000 (as depicted in Table 4.2).

4.1.5 Standard: Photo Scales for Nova Scotia Topographic Database Products

To adopt the photo scale for each map product indicated in Table 4.1 as the standard photo scale for that product, thereby allowing map compilation to achieve the necessary standard horizontal and vertical resolution.

* Source NSGC Data Development Section. All Nova Scotia base map products are representative of 90% of all data.

** Planimetric, Topographic and Digital Elevation Models

** The Nova Scotia Property Mapping Series is dynamic in that it is derived from a variety of sources. Each source will have associated accuracy and resolution values. Accuracy and resolution of this product must be determined based upon the sources used to present each property.

*** The 1:50 000 data provided here reflects files covering 4 map sheets in the East Hants Area as of Nov. 1995.

**** Source Natural Resources Canada - Sherbrooke, PQ [1995]. The specifications highlighted for these scales reflect federal government standards adhered to by the Province of Nova Scotia.

* The Nova Scotia Property Mapping Series is dynamic in that it is derived from a variety of sources. Each source will have associated accuracy and resolution values. Accuracy and resolution of this product must be determined based upon the sources used to present each property.

4.2 Topic: Data Accuracy Guidelines for Thematic Data Producers

4.2.1 Purpose:

To provide thematic data producers with guidelines on how to assist their clients in better understanding the intent and use of their data.

4.2.2 Background:

In the past, people or agencies were overjoyed to have access to ANY data which would help them in performing their geographic analysis. As these same people or agencies have become more versed in their analysis potential they have begun to expect more from the data. They not only demand better data, they are demanding a better appreciation of the data.

In January 1995, the Data Quality and Accuracy Task Group, under the direction of the Nova Scotia Committee on Standards for Geographic Information hosted a workshop on the issues surrounding data quality. In March the same year, the New Brunswick Geographic Information Corporation and the Nova Scotia Department of Municipal Affairs hired the University of New Brunswick to look into the two provinces' data structuring requirements for 1:10 000 scale base mapping. And while the data quality workshop and the New Brunswick/ Nova Scotia study were focused upon base mapping it was evident from the discussions that the participants were directing their concerns towards both primary and thematic mapping programs. In both exercises data consumers were asked questions regarding their understanding of the data they access. In most instances the consumers responded with an overwhelming need to better understand the data prior to its application. In particular they indicated a need to better understand the intent of the data set and subsequently the inherent accuracies of the data.

4.2.3 Guideline: Data Accuracy for Thematic Data Producers

Thematic data producers should provide their client(s) with a quantitative and qualitative indicator of the accuracies of the material being supplied. Whenever possible these indicators should be applied to individual elements within a given data set. In instances where such indicators cannot be stated for each element in the data set, the indicators should be applied to the feature/theme or, at a minimum, to the overall product.

To date there are no clear indications from the data consumer as to what is considered an appropriate data accuracy indicator. Suggestions range from specific numeric qualifiers to more general verbal descriptions. Examples of indicators which may be applicable, and their perceived effectiveness, are include below:

Textual-

Poor Accuracy Indicator
      "While this map may not be free from error or omission, care has been taken to ensure the
      best possible quality. This map is a graphical representation of property boundaries which
      approximate the size, configuration and location of properties. It is not a survey and is not
      intended to be used for legal descriptions or to calculate exact dimensions or area."

Poor Accuracy Indicator made somewhat better
      "While this map may not be free from error or omission, care has been taken to ensure the
      best possible quality. This map is a graphical representation of property boundaries which
      approximate the size, configuration and location of properties. Graphic representations for
      this map have been compiled from a variety of sources including:
           Assessment Mapping with survey not tied to control
           Survey Plans - Coordinated Survey
           Deed which was unspecified or unknown
      It is not a survey and is not intended to be used for legal descriptions or to calculate exact
      dimensions or area. The consumer is recommended to consult the specific features within
      this file to obtain a more realistic picture of feature sources."

Marginally Acceptable Accuracy Indicator
      "This data has been compiled from a 1993 windshield survey and was originally intended as
      a field check for a 1978 Agricultural field survey."

Numeric -

Very Good Accuracy Indicator
      "This data has been compiled from 1:50 000 stereo photography. Its horizontal resolution is
      3.0-5.0 meters for 68 % of all data and has a horizontal accuracy of approximately 6.0
      meters for 68% of all data."

What is important to notice in the examples provided is that disclaimers are not considered sufficient indicators of accuracy. Whenever possible data producers are encouraged to itemize how the file was compiled including types of source documents used.

4.3 Topic: Data Accuracy Statements for Digital Cartographic Products

4.3.1 Purpose:

To provide the user of digital cartographic products with a level of appreciation for the accuracy (and or limitations) of digital cartographic products.

4.3.2 Background:

Given the proliferation of digital mapping systems and the increase in desktop mapping applications in the geomatics industry there is an equal increase in the demands/expectations placed on digital data. The data providers should be aware of this increase as it has an effect on the demands placed on their specific data. A problem arises however when the client requests data from a provider and indicates in their request that they want to look at a cartographically appealing map on their desktop. All too often what results is the data provider gives the client either a non-cartographic data set (if for no other reason than it is the only digital product available) or a cartographic data set. In turn, unbeknown to the provider, the client decides to perform GIS analysis upon it at a later date.

The data provider must anticipate misuses of their data. This can be accomplished with very little effort if the appropriate data accuracy statement is attached to the data.

4.3.3 Standard: Accuracy of Digital Cartographic Products

In addition to the other accuracy standards found in sections 4.1 and 4.2 of the Standards Manual, data providers will also attach an accuracy statement to their digital cartographic products which highlight the limitations of applying full geographic analysis to the cartographic product.

Chapter 4 continued -
[Sections 4.4 to 4.7]   [Sections 4.8 to 4.9]   [Sections 4.10 to 4.11]   [Sections 4.12 to 4.16]  
[Sections 4.17 to 4.19]

Endnotes:

(1) The reader is encouraged to refer to the glossary of terms and terminology in Appendix A to determine the difference between accuracy and precision. Far too often these two terms are referenced incorrectly.

Chapter Four Table of Contents