Creating Base Maps With ArcGIS Pro
Base maps are reference maps used behind thematic maps to provide geographic context. While some thematic maps (like world or national-level maps) show areas that are recognizable by most users simply by their shape, state and local maps often need some detail in a base map so users can get a better sense of where the different areas are located in the real world. In addition, point maps and some line maps at all scales generally need base maps to avoid looking like a random collection of geometric shapes hanging in space.
A variety of base maps from ArcGIS Online are available for usse in ArcGIS Pro. However, these base maps are general-purpose, low-resolution maps designed for use with web maps, and there may be situations where you need to create your own base maps:
- Maps that will be displayed in large formats (like posters or high-resolution video) where low-resolution ArcGIS Online base maps will appear fuzzy
- Thematic maps where you need more or less detail than the standard base maps in order to be legible and/or useful
- Thematic maps where you need specific or specialized base information like environmental features, local political boundaries, or specific points of reference
This tutorial covers the steps for creating a base map, which are similar to those for creating a thematic map:
- Start the Map: Create a new project or add a map to an existing project
- Add the Thematic Features: Create a draft map over a standard base map so you can determine what is needed in the base map
- Add the Reference Features: Transportation lines, political boundaries, environmental features, places of reference
- Symbolize the Features: Create descriptive, clear, aesthetically-pleasing symbology and labeling
- Symbolize and Label the Features: Create descriptive, clear, aesthetically-pleasing symbology and labeling
- Present the Map: Create print layouts and / or publish as a reusable feature service
- Save Your Project: Save a project package to ArcGIS Online
Start the Map
To create a new map in a new project:
- Log in to ArcGIS Pro.
- When presented with options on the start screen, start your new project with a Map.
- Give the project a meaningful title so you can keep track of what is in different projects.
Add the Thematic Features
For this tutorial, we will use two example data sets to demonstrate different options for base map styling.
The example area data will be county-level electoral results for the 2012 US presidential election from the Minn 2016 Electoral Counties feature service from the University of Illinois ArcGIS Online organization.
The example point data will be a list of trauma centers from the Oak Ridge National Laboratories, that is distributed as a CSV file by the US Department of Homeland Security.
A trauma center is a hospital emergency room specially staffed and equipped to handle extreme life-threatening injuries like vehicle accident injuries, weapons injuries, major burns, etc. They are ranked in levels from I to V, with level I trauma centers being comprehensive regional facilities, with decreasing levels of capabilities down to level V centers that typically only stabilize patients before transfer to a higher-level center. (American Trauma Society 2020).
Add the Reference Features
A base map includes reference features that give geographic context for the thematic features and help map users understand where the thematic features are located in the real world.
Political boundaries define the extent of different governmental jurisdictions. In the United States, the general hierarchy of political boundaries from largest to smallest isis:
- Local Districts (wards, boroughs, city council districts, etc.)
The source for political boundaries in these examples is the TIGER Cartographic Boundary Files, maintained and distributed by the US Census Bureau. The cartographic boundary files are, as the name implies, designed for mapping and may not correspond with the exact legal juristidictions.
For a map like this example of central Illinois which covers the region of a state, the state, county, and city (place) boundaries are probably most appropriate.
Because we experience and interact with the real world via modes of transportation, roads are excellent reference elements, and can also aid in navigation if a user wants to travel to experience one of the areas specified in the thematic features.
The source for reference features in these examples is the TIGER / Line Shapefiles maintained and distributed by the US Census Bureau. Note that this is a different set of files from the cartographic boundary files.
Hydrographic features (rivers, lakes, and other bodies of water) are notable features of many communities that often give points of reference, and also affect the routes people choose to travel. Accordingly, they can be useful features on base maps.
The US Geological Survey (USGS) maintains The National Hydrography Dataset (NHD), which "represents the water drainage network of the United States with features such as rivers, streams, canals, lakes, ponds, coastline, dams, and streamgages." Because this dataset is so detailed, the USCB downloads are for individual counties. However, the USGS provides a feature service that can be added to a map. While this is a scale-dependent service that will not show features statewide, it is useful for smaller areas.
Symbolize the Reference Features
Symbology choices define a visual hierarchy that focuses the viewer's eye on what is most and least important on the map. This visual hierarchy is created through differences in visual weight created using size, thickness, color, and drawing order.
The definition of what is important is dependent upon the purpose of the map and the intended audience. For the map of level I trauma centers, if the audience is regional planners and / or public health officials who are assessing availability of trauma services to residents in Central Illinois. In that case, a visual hierarchy from most to least important could be:
- Roads: Access requires connectivity
- City names: Labels are helpful for quickly orienting readers
- City borders: These roughly show where clusters of people live together
- County borders: These might be important for assessing which local officials should be involved in further discussions
- Illinois border: This is effectively a neat line around the area of interest
- Other state areas: States outside the state of interest should be displayed since trauma centers in adjacent states may be part of the regional network, and they need context as well.
For the map of county election results, a potential audience could be political activists or operatives in an area who need historical information on past election results that will guide them on where to target limited resources in future campaigns. The visual hierarchy would be similar, although the county borders are part of the thematic symbols and do not need to be a separate layer.
- Roads: These indicate whether an area is rural
- City names: Labels are helpful for quickly orienting readers
- City borders: These roughly show where voters of people live together
- Illinois border
- Adjacent state borders: Needed only for context
The thickness and/or color of the lines specifying political boundaries should reflect their level of importance in the visual hierarchy. When mapping multiple levels of political boundaries, consider aligning the visual hierarchy with the political hierarchy.
When areas need to be higher in the visual hiearchy, you should consider using light, low-saturation fills to accent them. This technique may be more useful for point maps than area maps, where the shading will conflict.
For this example:
- Adjacent states are given a moderate gray fill and slightly darker border because they are only there for geographic context. They are at the bottom of the drawing order.
- County areas are made white to form the background of the area of interest. The layer is placed just above the states so it overwrites the gray state background.
- County borders are made very light gray to reflect their low priority.
- Cities are more important than counties in the visual hierarchy, so city areas are filled and bordered with a slightly darker shade of gray than the countes in order to stand out over the counties.
- The states layer is duplicated, a definition query isolates only Illinois, and then the state is given a thick, dark gray border to neatly enclose the area of interest.
Transportation features are often the most directly experienced and directly relevant features to map users. Accordingly, they should sit fairly high in the visual hierarchy, and should sit fairly high in the order of layers.
There are some commonly used line types for roads that are given as symbology options in ArcGIS Pro. You should probably use these options as fully as possible unless there is some reason to make other choices - such as the need for a low-detail base map that will not conflict with highly-detailed thematic layer(s).
- Within roads, there is also a hierarchy. In this particular TIGER dataset, the MTFCC parameter has two values: S1100 for interstates and S1200 for secondary highways. By symbolizing on this parameter, we can symbolize primary and secondary roads differently.
- For S1100 interstates we use a red limited access highway symbol that clearly stands out over the white background. Since this is a base map rather than a thematic road map, reducing the width to 2pt reduces the visual weight.
- For S1200 interstates we use a beige highway symbol that stands out over the white background and has less weight than the interstates, but more weight than the county lines. Again, 2pt width reduces the weight.
- The interstates should be drawn over any intersecting highways because the interstates are higher in the higherarchy. Selecting Symbology -> Symbol Layer Drawing and turn on Enable symbol layer drawing will make sure the items that are higher in the list of symbols will be drawn over lower items.
- Reducing transparency can also reduce the saturation and increase lightness so the roads do not dominate the visual hierarchy.
Label the Reference Features
In this hierarchy, county labels are low on the visual hierarchy and their visibility is not of great importance. To reduce conflict with other symbols and labels, the names labels labels are left black, but colored with 75% transparency so the more-important features will be visible under them.
There are a large number of small towns in Illinois, and labeling them all would make the map impossibly cluttered.
To label only features that meet certain criteria, select the labeled layer in the Contents pane, select the Labeling, and click the SQL icon on the left side of the pane. Here you can define conditions for which features get labeled. To label only physically large cities, we label only areas where ALAND (size of the city) is greater or equal to 30,000,000 square meters. The optimum value will be dependent on the particular area being mapped and the amount of detail desired by the cartographer.
The labels should be symbolized with a white Halo that will reduce conflict with features underneath the labels.
Although not demonstrated in this video, if you need to delete unneeded labels or move labels around, you can Convert Labels to Annotations to create an annotation layer.
Transportation features can be difficult to label because roads are often dense (causing label collisions), are represented in segments (causing redundant labels), and may not have name attributes that are too long for concise labeling.
By selecting the road layer and the Labeling ribbon tab, you have access to options to automatically adjust the labels.
- In this example, because the map covers a large area, we use an SQL statement that only displays labels for highways that have names that start with "I-"
- US interstate numbers are commonly included blue and red symbol called a shield.
- Redundant labels in adjacent sections of interstates can be removed by selecting Label Placement Properties -> Position -> Remove duplicate labels.
- The Text Symbol size can be reduced if the names don't fit in the shield.
- Changing Position -> Placement to Offset horizontal will keep the shields upright rather than following the direction of the highway.
Symbolize the Thematic Features
Once you have basic styling of your base map, you can add the thematic features and adjust the symbology so that the features are clearly visible over (and contextualized by) the base map information (figure-ground relationship), and so the visual hierarchy best suits the purpose for the map.
Point Thematic Symbols
Thematic features are generally considered the at the top of the visual hierarchy in a thematic map. Accordingly, they are also usually layered on top of the base map features.
- You may want to duplicate your base map and use the duplicate for thematic mapping so that you can preserve your original choices for use with other maps. Publishing the base map as a reusable layer (described below) may also be useful if you plan to reuse this layer for additional maps.
- The heart pictogram that looked ok with the neutral topographic base map now gets lost with this more detailed base map. Changing the color to a simple dark blue dot addresses this issue, although it is not particularly pretty.
- If labels from other layers overlap your features, you need to increase feature weight under Labeling -> Map -> More -> Weights. Feature weight is used by the labeling algorithm to place labels in appropriate locations.
- On the whole, the base map as created works fairly well under this point map. The removing the interstate shields avoids a conflict with the blue thematic points.
Area Thematic Symbols
Because thematic areas are often inherently large and less detailed that base map features, there may be occasions where thematic areas should sit under the base map features. You may then need to adjust the base map feature symbology and/or the thematic area symbology to match the desired visual hierarchy.
- Moving the thematic layer above the counties will hide the white background, but leave the county labels.
- The significant number of cities makes the background too busy. Making the cities transparent will leave the city labels, which might be useful.
- The black city name labels are now lost on the dark thematic symbols. Changing them to white letters with dark halos remedies this.
- Likewise, the secondary highways are too busy against the dark, saturated. Setting a definition query to leave only the interstate highways mitigates that problem.
Present the Map
Publish the Base Map As a Feature Service
If you plan to reuse your base map in the future, and especially if you want to share your base map with other users in other projects, you might consider publishing your base map as a web layer.
Remove all the thematic symbol layers and any unused layers before publishing.
One issue that may arise is that some of the more complex symbology and labeling options (like complex road symbols, highway label shields, and custom symbol layer line drawing order) cannot be saved to feature layers. You will be notified with warnings when you analyize your map before sharing, and you can ignore these warnings if simplification of your symbols will not be a problem for you.
Once we have gotten the map content the way we want it, we create a new Layout that will allow us to design how the map will look when inserted into a document as a figure.
- Insert a new Print Layout. For this example we will assume we are creating a 4" x 6" figure which will flow nicely in the text of a report on standard 8.5" x 11" paper.
- Add a Map Frame for the map you created.
- Right click on the frame, select Properties, Size the frame to fill the entire figure and Position the frame at the edge.
- Remove the black frame around the map frame since it might not come in cleanly when the map is inserted in the document.
- If you need to move the map inside the map frame, right click on the map and Activate the map so that dragging and zooming affect the map in the frame rather than the map layout.
- When you are done adjusting the map, click the arrow at the top of the window to deactivate the map.
- Add a North Arrow.
- Share and Export the layout to a PNG file. Portable Network Graphics (PNG) is a type of file that, unlike JPEG files, does not degrade the quality of the image to reduce file size. For small figure images, file size is not usually a major issue.
- You can then Insert the PNG file as a Picture in a Word or Word 365 document.
- Give the figure a meaningful caption that includes the source of the data.
Standalone Print Layout
You can also create print layouts that are ready to print directly. Cartographic convention dictates adding some additional marginalia information to the margins of printable maps.
- Insert a new Layout. For this example we choose a standard 8.5" x 11" paper size. If your map is wider than it is tall, choose Landscape orientation. Otherwise, choose Portrait.
- Insert a Map Frame. Leave some space around the edges for a margin, and center the map on the page.
- Insert New Text icon, drag a box where you want the title, and type the title into the text box. The box will expand to fit the text, and you can drag the edges in to resize the text.
- Add a text box for the standard Marginalia:
- The cartographer (you)
- The date the map was created
- The source for the map data
- Add a Legend. These can go inside or outside of the main map frame.
- Add a North Arrow to show the reader where north is on the map. While north is normally up on maps, that is not always the case, and an orientation icon is a safe practice These come in a variety of styles and can add some aesthetic flair to the map.
- Add a Scale Bar.
- Finally, Share and Export a PDF of the map that you can print. Portable Document Format (PDF) is a type of file developed by Adobe that preserves the formatting of a document as you created it so that documents appear the same on different computers or printers. This is the preferred output format for printable maps.
Save Your Project
Saving a Project Package
When you are done with a project, you should save it as a project package on ArcGIS Online:
- This package will save all of your maps, layouts, and local data together so that you can reopen the package later on any computer if you need to modify or recreate any maps.
- You can share the package in case you want someone else to be able to use the materials you created in the project.
- Saving this to ArcGIS online will protect you from losing your data if something happens to your normal work computer.