Creating Map Symbology Learning ArcGIS Desktop Training Course

Creating Map Symbology Learning ArcGIS Desktop Training Course
Module 2 ESRI Virtual Campus Learning ArcGIS Desktop Training Course ESRI ArcGIS Section 1

Creating Map Symbology
Have you ever noticed that some maps are easier to understand than others? Difference can be due to mapmaker's choice and arrangement of symbols and text. A map is most effective when symbols are easy to distinguish and their meaning is intuitive. Your choice of symbols and labels will be influenced by the type of map you are making. Section 1

Creating Map Symbology
Maps can be divided into two main categories: Reference maps Also called general maps Show the location of features Useful for multiple purposes Examples - atlas maps and topographic maps Thematic maps Show the structure and distribution of one or more phenomena Examples -maps of world population, today's weather, and rice production in the Philippines Section 1

Learning Objectives Choose symbols for point, line, and polygon features. Modify symbol properties such as color, size, and outline. Label map features using an attribute and by adding text. Symbolize features to show type, rank, or amount. Group features into classes and apply symbols to each class. Compare different methods of grouping features into classes. Correct visual distortion caused by differences in area. Show proportional amounts on a map by normalizing data. Symbolize features to show density. Section 1

Working with Map Symbols and Labels
When a layer is added to a map a default symbol is used to represent the layer's features. It may not be the one you want, so you need to know how to change it Effective symbols take advantage of common associations that people make, such as blue for water and green for vegetation. People also make associations based on symbol size street drawn with a thick line is easily understood to be busier or more important than one drawn with a thinner line Text may be used to provide a feature's name or other attribute, or to draw attention to a feature or an area of interest. Section 1

Working with Map Symbols and Labels
On a map, symbols are used to show feature locations. Using pictoral symbols can provide more information i.e. a car symbol indicates a parking lot. Adding text such as a feature's name or function provides even more information. Section 1

Types of Symbols Symbol properties can be changed to suit a particular map. i.e. can change the size or angle of a marker symbol used to represent a point feature Can change the width of a polygon symbol's outline Section 1

Symbol Properties Section 1

Types of Symbols When a map document is saved Layer file
Layer symbology is saved with it. Layer file To easily reuse a layer's symbology in other map documents A good way to ensure that everyone in an organization uses the same symbology Can be especially important if organization or industry employs standard symbols Can also save time --- don't have to create the symbology over again each time a particular layer is used Section 1

Choosing Symbols ArcGIS has thousands of symbols for common map features Organized into more than 25 symbol sets Can also import additional symbol sets or create your own General-purpose symbol sets Like the ESRI symbol set Specialized symbol sets Reflect the needs or standards of different industries Section 1

Choosing Symbols A few of the point (marker) symbols that come with ArcMap. From ESRI, Crime Analysis, Utilities, and Forestry symbol sets Clockwise from upper left Section 1

Labeling Map Features Text on a map can serve many purposes
One of most important functions of map text is to describe, or label, features. Most common labels are names i.e. street names, place names, and names of landforms or water bodies In ArcMap, label text comes from a field in a layer's attribute table. Section 1

Labeling Map Features The source of the labels in this map is the CNTRYNAME field in the Countries attribute table. Section 1

Labeling Map Features Labels can be added in two ways Dynamically
Generated on the fly for all the features in a layer at once Can specify label properties such as font, size Color position in relation to the feature being labeled (such as top left, bottom center, top right Interactively Created by clicking individual features in the map May use same label properties specified for the layer, or can set different ones Section 1

Labeling Map Features What if you want to label something that isn't actually represented in a layer? i.e. a layer of mountain peaks and you want to label the whole mountain range. You can do this by manually adding the desired text to the map. Section 1

Labeling Map Features Annotation Dynamic label Text added manually
Each piece of annotation has its own properties that are stored either in a map document or in a database Always stays at the position where you place it, but you can reposition it as desired Dynamic label Location is determined by ArcMap and is based on the current map extent and the number of features being displayed in that extent. As the map is zoomed in and out, the position will change as ArcMap determines the best placement for them. May move, appear, or disappear as the available space on the map changes. An easy way to label many features at once. Can convert dynamic to annotation if you need to edit the appearance or placement of individual labels. Section 1

Exercise Display and label map features Section 1

Symbolizing Features Based on Attributes
Features can also be symbolized based on an attribute Thematic map Features have been symbolized based on an attribute Often communicate more information. i.e. vegetation polygons symbolized by a type attribute to indicate different areas such as forest, grassland, or marsh. Individual tree locations could be symbolized by a diameter attribute to show the distribution of trees by size. Section 1

Pine trees have been symbolized based on their diameter Vegetation polygons have been symbolized based on their type. Now you can see that the largest trees occur in just one cluster. You can also see that pine trees are not found in marsh areas. Section 1

Type of symbology used to create a thematic map depends on whether an attribute's values are categories (e.g., type) or quantities (e.g., diameter) Section 1

Drawing Features to Show Categories
Category attributes are Names Types Ranks Each unique attribute value represents a different category. The values can be Text Numbers Section 1

Section 1

When a layer is symbolized based on a category attribute Features in different categories are represented with a different symbol. Exactly how the symbols differ from one another depends on is being mapped i.e. if symbolizing roads according to the number of lanes use line symbols with different widths to represent the different lane categories If mapping roads according to surface type show paved roads with a solid line and gravel roads with a dashed line Section 1

Two ways to symbolize a hurricane path by categories Left different colors are used to symbolize the paths by name Right different line widths are used to symbolize the paths by hurricane strength Section 1

Drawing Features to Show Quantities
Quantity attributes are always numeric. Numbers represent Counts Amounts Rates Measures Section 1

Feature quantities typically represented by Creating groups of features with similar values (classes) And assigning a different symbol to each class. Even though symbols are different usually change gradually from one class to another forming a series. Graduated size and graduated color are two most common ways to symbolize quantities Section 1

Drawing features using symbols in a graduated series permits map readers to visualize geographic distribution patterns in quantity data. i.e. if a map is drawn with colors ranging from yellow to orange to red red areas can quickly be interpreted to represent greater values than yellow ones Likewise, smaller symbols represent smaller quantities than larger symbols. Section 1

The countries in this map are displayed with graduated shades of green. The darker the shade, the greater the country's population. Section 1

When choosing graduated colors, it is important to be aware of common color associations that people make. People will easily understand a temperature map drawn with blue symbols for cold orange symbols for warm The opposite way would be frequently confused. Section 1

Exercise Display features with categories and features Section 1

Classifying Data What process is used to create the classes?
What determines whether a particular attribute value falls into one class or another? When symbolizing features based on quantities - three things must be decided: How many classes to have What method to use for placing the values into classes What kind of symbology to use e.g., graduated colors or graduated symbols Section 1

Classifying Data This graduated color map was created by classifying population values into four classes. Section 1

Grouping Attribute Values Into Classes
In ArcMap, can classify features using one of several standard classification methods Can also define own classes. Section 1

Classification methods include: Natural breaks Identifies groupings of values that are inherent in your data. Is the default method because it is appropriate for most data. Equal interval This method is like a ruler: the interval between each class is the same. i.e. you might have classes with intervals of 10 percent (1-10%, 11-20%, 21-30%, etc.) Section 1

Quantile each class contains an equal number of values (features) i.e. you might have 15 provinces grouped into three classes Each class would contain five provinces regardless of the attribute values. Manual each class has the range you specify This method is useful when you want the classes to reflect specific criteria or data. i.e. if you have temperature data, you might want to specify a break between classes at 32 degrees Fahrenheit (freezing point). Section 1

Natural Breaks uses a statistical formula to find breaks that are inherent in the data. In this example, there is a clear natural break between 19 and 29 (a difference of 10), but not between 29 and 30 (a difference of 1) Section 1

Equal Interval Evenly divides the entire value range into the number of classes you choose. Section 1

Quantile Places an equal number of values into each class. Section 1

Manual Uses class breaks that you define. Section 1

Deciding Which Classification Scheme to Use
When mapping quantities, you may ask yourself: Which classification method should I choose? How many classes should I have? There are no "correct" answers. The best classification scheme for a given map layer depends on Purpose of the map Characteristics of the data Cartographic considerations such as how easily the resulting map can be understood Section 1

One approach let the data inform your decision. When looking for patterns in data try different classification methods and visually analyze the resulting maps then select the method that seems best. To evaluate classification schemes before mapping them use a histogram Section 1

The classification histogram charts the number of attributes (features) for each attribute value. Bottom axis shows attribute values Side axis shows frequency of values Height of gray bar indicates number of times a given value occurs in the table (frequency) When deciding on the number of classes, there is one rule of thumb you can use: Fewer is generally better Three to seven classes is usually best. Section 1

A classification histogram helps visualize how attribute values are distributed across the overall range of values Blue lines show current class breaks Highest attribute value in each class Data in this histogram is grouped into three classes. Section 1

Another approach choose a classification scheme first And let the attribute values fall where they may. There may be a scientific or statistical reason for using a particular classification method with particular data. Or, you might have predetermined standards or criteria that dictate the method or number of classes. Section 1

General guidelines for choosing an appropriate classification scheme. Section 1

Exercise Explore methods of classifying data Section 1

Mapping Density and Proportion
Sometimes mapping an attribute with graduated colors or symbols can be misleading. i.e. when polygon features vary greatly in area. Patterns may be perceived in a graduated color map and assumed to represent variation in the attribute being mapped, when in fact they reflect the variation in the area of the features. You can avoid such misperceptions by mapping density—the quantity per unit of area. Section 1

Example Each polygon represents a pasture in a goat farm. The small pastures are each 1 hectare and the large one is 4 hectares in area. This map shows the goat pastures This map seems to show that goats are concentrated in the north half of the farm. There are actually more goats in the south half of the farm. Mapping density results in a map that is likely to be perceived correctly. Section 1

Another situation is when mapping the proportion of one quantity to another is more important than mapping them individually. In the goat farm example, the proportion of female to male goats in each pasture might be more important than the total number of goats. Section 1

Mapping Density Using Attribute Values
One way to map density is data normalization Divide the attribute values by the area of each polygon feature ArcMap calculates the density values Choose a value field and an area field You still must choose a classification method for grouping the density values and symbolizing them with graduated colors or graduated symbols Section 1

When normalizing data by area make sure attribute field contains counts or amounts i.e. population, bushels, or number of species Be aware some attribute fields store values already normalized i.e. population density or bushels per hectare These should not be normalized again Section 1

The Under_18 attribute which contains the number of people under age 18 is normalized by area in order to show the density of children The default classification method was used natural breaks with five classes Section 1

Mapping Density Visually
Dot density map mapping density visually by using dots to represent quantities of things in the real world. Each dot represents a specific count or amount. The more dots in an area, the greater the quantity. The closer the dots are together, the greater the density. It's important to understand that ArcMap places the dots on a dot density map randomly within each polygon feature. Unlike symbols representing a point layer, the individual dots in a dot density map are not associated with actual location coordinates. Section 1

Mapping Density Visually
A dot density map provides a visual sense of the density of objects or quantities in the real world. Section 1

Mapping Proportion Can map relationship between two attributes by normalizing (dividing) one by the other to produce a ratio. As when mapping density, normalized data is typically symbolized using graduated colors or symbols. Section 1

Mapping Proportion Section 1

Mapping Proportion Three common ways to represent proportions in a legend: Ratios Range between 0 and 1 Percentages Ratios multiplied by 100 Rates based on a round number i.e. per capita (per person), per 100, or per 1,000 Section 1

Mapping Proportion Ratio of children to adults
mapped by normalizing one attribute by another Two alternative legends are shown: Top = ratios Bottom =rates Section 1

Exercise Map density and attribute relationships Section 1

Review Point, line, and polygon symbols have properties that you can set, such as shape, size, color, outline, and width. Effective symbols take advantage of common associations that people make, such as blue for water or a larger dot for a more populated city. Symbolizing features by attributes allows you to communicate more information. You can symbolize features to show categories (names, types, ranks) or quantities (counts, amounts, rates, measurements). Quantity attributes can be classified using different methods, including natural breaks (the default), quantile, equal interval, and manual. Which classification scheme you choose depends on the purpose of the map and the characteristics of the data—there is no one "correct" choice. Section 1

Review Questions When you label map features in ArcMap, where does the text come from? When classifying a layer, what rule of thumb can you use to decide how many classes to use? Name two things you can learn from a classification histogram. Name two ways that density can be symbolized on a map. Section 1

Review Answers In ArcMap, label text comes from a feature attribute or you can manually add your own text to a map. When classifying data, fewer classes is generally better. You can learn many things from a classification histogram. Your answer could have been any of the following: How attribute values are distributed across the whole range of values The minimum attribute value The maximum attribute value The number of classes The class breaks (maximum value for each class) The size of classes relative to one another The number of features that have a particular attribute value. You can show density on a map by normalizing an attribute by area and using graduated color or graduated size symbols; you can also create a dot density map. Section 1

Creating Map Symbology Learning ArcGIS Desktop Training Course

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