SCALE & ACCURACY.

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SCALE & ACCURACY

SCALE The amount of reduction that takes place in going from real-world dimensions to the new mapped area on the map plane. Defined as the ratio of map distance to earth distance, with each distance expressed in the same units of measurement.

Ratio & Fractional Scales
Expressions of Scale Ratio & Fractional Scales 1 1:24,000 24000 Equates one unit of distance on the map to the corresponding distance on the ground.

Verbal Scales One foot equals 24000 feet One inch equals one mile
Useful for a quick sense of ground units in familiar units. Unreliable, subject to misinterpretation, invalidated by reduction and enlargement.

Bar Scales Most effective
Map user can better measure and interpret distances within the map area. Expands or shrinks along with other map distances, so it remains valid over all reductions and enlargements.

Large Scale vs. Small Scale
1 Large Scale e.g. 1:1000 1000 1 Small Scale e.g. 1:250000 250000 The terms “large scale” and “small scale” refer to scale shown as a fraction. 1:1000 is a relatively small denominator, yet it is a much bigger fraction (and thus a larger scale) than 1:250000 Large scale map features are relatively large. Small scale map features are relatively small.

Large Scale Shows a relatively small portion of the earth’s surface
e.g. 1: quad scale e.g. 1: quarter quad scale e.g. 1: tidelands maps Shows a relatively small portion of the earth’s surface Provides detailed information Usually maps that are 1:24000 or larger are considered large scale

Small Scale Shows relatively large areas of the earth
e.g. 1:250,000 - Hudson county e.g. 1:3,300,000 - State of New Jersey Shows relatively large areas of the earth Provides limited detail Generally maps smaller than 1:24000 are considered small scale.

Forms of Geographic Phenomena
The form is closely related to scale and may change with the level of inquiry. At a small scale a point or line may be sufficient. At a larger scale a polygon may be more appropriate.

Large Scale Small Scale

ACCURACY All maps are generalizations
Too much detail for the sake of precision could overwhelm an audience Too little attention to detail could result in inaccuracies that misinform or anger an audience An accurate map is one that communicates the message clearly, and helps the audience understand the limitations of the data.

Most critical for larger scale maps.
Geometric Accuracy The distance between the actual location of a feature and its mapped location. Most critical for larger scale maps.

1:100,000

Geometric Accuracy (cont.)
Small scale data printed on large scale maps can be coarse, oversimplified and misleading. When digitizing from older map sources, note the risk of compounding original error with new error. If digitizing manually make proof plots. Note accuracy of data creation techniques: Addressmatching +/- 200 feet GPS +/- 15 feet

Inaccuracy in source data that cannot be corrected
Factual Accuracy Inaccuracy in source data that cannot be corrected Use text to explain the factual limits of the map. Especially when ... Map features have changed since data was created, and it’s not practical to update it. Too much generalization exists, such as when the data was compiled at a smaller scale than the map. Metadata is a good source of info related to factual accuracy.

Accuracy in Classification
Classification is a form of generalization, therefore it affects map accuracy. Experiment with different classifications before deciding which one both accurately represents the data and suggests proper interpretation. Be careful of mapping source data whose classification does not exactly fit topic of map.

Accuracy in Interpolation
Interpolation is the process of deducing the unknown values that occur between points with definite values. e.g. isolines, addressmatching.

Accuracy in Interpolation (cont.)
Inform audiences that they must take care in interpreting the data. When interpolation is especially sketchy, or if a feature is incomplete, use dashed or dotted lines to relate its approximate nature to the audience.

Accuracy in Interpolation (cont.)

Accuracy in Interpretation
The potential for misinterpretation by map reader Address possible uncertainty in the interpretation of data,symbols and labels. e.g. Pollution concentrations just above a standard might seem much more dangerous than ones just below the standard. Non-detectable levels may be interpreted as zero and seen as far safer than only slightly higher detectable levels.

GIS Spatial Data Standards
FGDC standards have been published for documenting and testing the spatial accuracy of GIS data. National Mapping Accuracy Standard 1994 FGDC Metadata Standard 1998 FGDC Geospatial Positioning Accuracy Standard 1998 NJDEP Electronic Data Submittal and Interchange Standard

National Map Accuracy Standards
Geometric Accuracy of published maps,especially federal topographic and photographic base maps NMAS Horizontal > 1:20,000 not more than 10% test points in error by > 1/30” < 1:20,000 not more than 10% test points in error by > 1/40” 1:12, ft 1:24, ft 1:100, ft NMAS Vertical not more than 10% test points in error by more than 1/2 contour interval

Why a new standard is needed
NMAS focus on the accuracy of paper maps The same processes that make digital cartography so powerful for data analysis and representation complicate accuracy assessment: Errors can be introduced by scale variability, digitizing source materials, processing algorithms, photogrammetry, resolution, and peripheral limitations. Collective error propagation can result in data that are oftentimes inappropriately used Decisions made on data of known quality are made with greater confidence and are more easily explained and defended.

FGDC National Standard for Spatial Data Accuracy
Geospatial Positioning Accuracy Standards NSSDA specifies a positional accuracy test based on comparing the locations of at least 20 points between a test theme and a reference theme of higher precision. Horizontal & Vertical Accuracy is expressed as a function of the distance error (Root-Mean-Squares Error) between the two themes (a check and reference theme) in the x- and y-coordinate directions for horizontal, and z-coordinate direction for vertical. Threshold values are unspecified and left to the discretion of organizations

FGDC National Standard for Spatial Data Accuracy (cont.)
Suggested horizontal accuracy for large-scale maps by the American Society of Photogrammetry & Remote Sensing (ASPRS): Scale Feet 1: : : : : : :1, :2, :4, :6, :9, :12, :20,

ArcView Extension (RMSEr2
ArcView Extension (RMSEr2.avx) Implementation of the NSSDA Test Procedure Written by N.J.Geological Survey and distributed on website Example on left shows the display of test results for a horizontal accuracy test between a 1:100,000 scale DRG base image and a DLG Hypsography theme with two sets of points checked.

LAYOUT

Cartographic Elements
. Subject Area . Title . Legend . Scale . North Arrow . Tables or supporting graphics . Border or Neatline

Rabies Vaccine Field Test Mapping

Subject Area The main subject should take up as much space as possible.

Title Should clearly convey the map message
Should be direct and concise Subtitles in smaller type can qualify or expand the statement in your title Avoid jargon that is unknown to audiences Avoid using the word “map” in your title Choose a text font that is prominent and readable

Legend (key) Describes the graphic code delivering the message
Symbols should look like counterpart on map Can warn of uncertainty in data Try to create symbols that don’t need a key Use a descriptive legend heading whenever possible.

Legend (cont.)

Legend Maker Extension on the tool bar that allows you to add a customized legend to your layout Displays only those features on the layout that appear in the View. Provides more flexibility in the way symbols are displayed.

without Legend Maker

with Legend Maker

Scale Can use a combination of bar and verbal or bar and ratio scales.
Should not be overly large, easy to read but not prominent. Place in a recessive location

North Arrow Points in the direction of true north
Mandatory if north is anywhere except the top of the page Keep small and unobstrusive

Tables or Graphics Tables Photos Floor plans Charts Statistics
Enhance map info. But don’t overwhelm the map reader.

Border Also known as a neat line or map frame
Defines boundaries and enhances map reading Strongly recommended for a formal presentation

Typography You must be concerned with both the content and the form of the text. Keep all text brief but accurate Make sure text is legible Use no more than four fonts or text sizes on a map. Beware of using decorative type

Typography (cont.) Use plain text for land,italics for water
Spread the name out along the feature

Default labeling style

Balloon labeling

Position of Map Elements
Subject Area should be large, and take up the center of the map. Consider placing the most important cartographic elements near the top or left of the map. Less important and supporting map elements can be positioned toward the bottom and right.

Position of Map Elements (cont.)
Title should be at the top, usually the center but sometimes to the side. North Arrow and Legend should be to the side at the middle or bottom. Scale usually near the bottom. Use a style sheet when a series of related maps are being made, especially by different people.

Map Balance Minimize white space by making the subject area as large as possible. Title, legend, scale and north arrow should be graphically subordinate to the subject area. Balance map elements (imagine hanging a picture). Use the “Size & Position” option under the Graphic Menu to help balance the map.

Example of Map Balance

Templates Streamline map production Ensure consistency of design

Create a Template from your Layout
The View becomes the layout title. The View and Legend frames are updated on each consecutive layout

Save a Layout as a Template
From the Layout menu, choose Store as Template. The Template Properties dialog appears. Type in a name for your template and select an icon for it. Press OK.

To choose a template for a layout
1. From the View menu, choose Layout. 2. In the Template Manager dialog, click the template you wish to use. 3. For more information on the use of templates, follow the instructions for Style Sheets in the “Design Guide for Environmental Maps”.

CONCLUSION Follow guidelines as much as possible
Don’t be surprised if you have to break the rules once in a while. Have your map reviewed by people less familiar with the subject. Always remember your message