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
- Acquire the Data
- Symbolize the Features
- Label the Features
- Present the Map
- Save the Project Package
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.
Acquire the Data
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 is:
- County (Parish in Louisiana)
- 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.
- Go to the USCB's Cartographic Boundary Files - Shapefiles page.
- Download the appropriate shapefiles. For a map like this example of llinois, the state, county, and city (place) boundaries are probably most appropriate.
- Extract All the shapefile contents from the .zip archives.
- View the Catalog Pane.
- Import the shapefiles as feature classes into the project database. This step is needed to assure that the features are preserved when the project package is saved.
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."
NHD data for individual states is available from NHD View.
The shapefile dataset contains many files that are described here. For Illinois, these seemed to be the most useful:
- NHDWaterbody: Minor lake/pond features
- NHDWaterbody2: Major lake/pond features
- NHDArea: Additional water polygons
- NHDFlowline: Streams, rivers, and canals
For this tutorial, 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).
Symbolize the 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:
- Thematic features (trauma centers)
- Roads: Access requires connectivity
- 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.
The TIGER data is in unprojected NAD 1983 geographic coordinates. Use of a Web Mercator projection is a safe cartographic choice.
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:
- States are given a moderate gray fill and slightly darker border because they are only there for geographic context.
- Illinois (STATEFP == 17 for Illinois) is removed with a definition query and the states are moved to the top of the drawing order so they form a mask to clip any features that extend beyond the border of the state (notably counties and hydrography).
- County areas are made white to form the background of the area of interest.
- County borders are made very light gray to reflect their low priority.
- To limit the number of cities displayed, a definition query is added to limit display only cities with an ALAND (land area) of >= 30,000,000 square meters.
- 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.
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 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, 1pt 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.
Unless you are specifically creating a base map for environmental phenomena where hydrographic features should high in the visual hierarchy, hydrographic features are generally low in the visual hierarchy and are layered below transportation features (bridges go over water) and political boundaries (which often cut through the middle of lakes and rivers).
As with transportation features, hydrographic features are generally filled with some color evoking water (such as a shade of blue). However, as with the shading of political boundaries, experimentation may be needed to find a symbology that does not visually conflict when used with choropleth layers.
Hydrographic features can have fuzzy borders that change with water levels, and symbolizing them without line borders can both keep them lower in visual heirarchy and represent that ambiguity.
Example Thematic Features: Trauma Centers
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.
- Because the base map is complex, a simple dark blue dot has maintains a good figure-ground relationship, although it is not particularly pretty.
- A definition query subsets only level I trauma centers in the analysis state.
- Placing the layer above the state mask layer avoids the clipping of features in border cities.
Label the Features
There may be situations where you want to label the base map features. With this base map, labels may only be appropriate if you are mapping a portion of the state, or if you remove some of the 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.
City 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.
Present the Map
The following video shows how to create a stand-alone map that can be printed to 8.5 x 11" paper.
- 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 so that you can reopen it later on any computer if you need to use your base map.
- Go to the Share tab and select Project.
- Provide a name to save the project under. The default is the name of the current project.
- Copy the name into the Tags and Summary fields.
- Click the Share outside of organization box so your project database containing all of your layers is included in your project package.
- Unclick the Include Toolboxes and Include History Items check boxes so that history or toolbox errors to not cause your upload to fail.
- Analyze the project to find any problems.
- Package the project to upload it to ArcGIS Online. This may take a minute or two.