Map Basics
Now we see a box representing the color axis of the map layer. Low values are beige in color and high values get more rust-colored. The maximum appears to be 1,576,390. But 1.5 million what? What exactly is being shown? This helps a bit, but lets see what else we can learn.
3.4. Click the three blue dots under CEISEN Population to expand the drop down, and then click Description. A new browser tab will open up.
This page contains the Metadata for the layer; that is, this is data about the data. It should tell you who created the layer, the source of the data, the type of data (vector/raster,etc), the spatial extent, and other information. Data without metadata is virtually worthless. Examining the information, we see that this is a raster layer which shows the population density per square mile and is based on a variety of census and demographic data. Let's find some more pertinent data and add it to the map.
Tip: The description of a layer can be opened in the Search tab to the map to assess its usefulness before you add the layer .
3.5. Click the three blue dots under ESRI Population World to expand the drop-down and click Remove.
3.6. Find the layer USA NLCD Land Cover 2011, click its name in the search bar, and in the bubble, click Item Details.
The details for this layer are loaded. Once you've taken a look at them:
3.7. Click the Open button and Add layer to map
3.8. Click the Details tab and the Legend button to display our map legend.
Now we can see that this layer contains pixels of land use classification from a variety of categories.
4. Attribute tables and layer symbology
Part of the power of a GIS comes from the ability to create maps which tell a story, and much of this is the way in which the data are symbolized and represented. Remember that all the data in a layer is kept in a database (think spreadsheet) and the GIS is simply displaying that information in a useful way. Let's look at the attribute table for some data and change how its displayed.
4.1. Find and add the layer USA Major Cities. Navigate to the table of contents.
4.2. Click on the Show Table button to display the attribute table.
This is the table of contents for our map. The layers are listed in the order in which they are displayed, from bottom to top. So far, we have our basemap (topographic, imagery, or other) on bottom and this layer, ESRI Population World, above it. Let's examine the legend for this layer.
3.2. Click the black arrow next to the layer name to expand its contents.
3.3. Hover the mouse under the word CEISEN Population and click the Show Legend button.
You should now see a map showing areas of high population density.
3. The Details menu
So we added a layer that appears to show population density. What do the colors mean? Who created the layers? Where did the data originate? We need to look at a few things to understand our data. Let's first try to understand what is being shown.
3.1. At the top-left of the page, click Details, and if not already selected, click Content.
2.2. In the Add drop-down, click Search for Layers.
2.3. In the Find box, type population density and in the In: box, select ArcGIS Online.
2.4. Find the layer named ESRI_Population_World and click Add.
2. Adding data to the map by searching ArcGIS online
An enormous amount of data is already available on ArcGIS online and in the Living Atlas layers. Some of these datasets contain attribute tables and others do not. Much of the data you may want to use can be found using the Add tool.
2.1. Click on the Add button to expand the drop-down.
There are options to search online, search the Living Atlas layers, import from a file or from the web. As an example, let's look at population density across the globe.
From here, we will see what kinds of basemaps are available. The Basemap is the bottom-most layer in our online map. Every other dataset we add or create will display on top of the map. You may want to change the basemap based on what story you are trying to tell with your data.
1.4 In the upper left of the window, click on the Basemap button to expand the drop-down menu.
A number of different styles are available. Topographic is the default and already displayed. These maps show ground surface topography and a mix of earth features (streams, lakes, wetlands, forests, etc.) as well as features like cities, roads, and borders.
1.5 Click Imagery
This basemap is a mosaic of aerial photographs showing how things appear from above, like on Google Earth. Other maps are available, explore what they contain and think about why you might want to use one or another.
Here we will show some basic features of a GIS with which you should be familiar. Once you are logged in you, will be on your ArcGIS Online home page. You should see a banner with the name of the organization you created. From there, we will create a new map and explore some features and tools available to us.
1. Creating a new map and changing the basemap
1.1 To begin, click Map on the top of the homepage. This will create a new map for you with the default world map.
1.2 Enter a name for your map: Yourname_DemoMap.
1.3 Each map must have tags, so enter watershed as a tag.
The table tells us a number of things: there are 3,557 cities in the layer, and for each city are several attributes, including a ranking (Pop. Class) and the 2007 population and many others not initially displayed. Notice that right now, all the points are displayed in the exact same way on the map, but the points in the dataset can be changed to represent the information in these attributes. Let's change the symbology of the points based on the 2007 Population attribute and see what is available to us.
4.3. Click on the Change Style button. A menu will appear.
The menu will change the symbology based on the attribute selected in the first drop-down. First, let's try the Counts and Amounts (size) option, which will change the size of the points according to a value. 4.4. Select the Population (2007) layer at Step 1.
4.5. Click the Options button under Counts and Amounts (size).
Here we can change the minimum and maximum sizes of the points and the symbol color and type. We can also classify our data into discrete groups based on the attribute, and display classes the same way.
4.6. Click the circle above the word Symbol and select a Symbol, Fill color, and Outline color you like. Click OK.
4.7. Click the checkbox next to Classify Data. Leave the type as Natural Breaks and change the number of classes to 6.
A number of classification methods are available, and you can explore them more on your own. Natural Breaks classifies the data according to, well, natural changes within the dataset, and is a good place to start. Your map should now show the cities with variably sized icons. This may be a good choice for certain data sets.
Now let's change the color of the points to represent the population.
4.8. Click cancel to go back to the symbology menu.
4.9. Select Counts and Amounts (color)
The dots are now the same size and displayed with a color scale to indicate the population. The default color scale isn't very useful, so let's change it:
4.10. Click the color ramp icon above the word Symbols.
4.11. On the Shape tab, change the symbol to 15 pixels.
4.12. Click Fill and select the very last color ramp, going from deep red to yellow. Click OK.
The dots are gradually changed in hue to represent population.
4.13. Try classifying the data with different numbers of classes to see how the map changes. Note that the available color ramps change when the data are classified vs continuous.
4.14. Click cancel to go back to the symbology menu.
Next, lets use what's called a Heat Map to show the population centers of the country. A Heat Map will use a color scale to show very high values.
4.15. Select Heat Map and click Options.
4.16. Play with the Area of influence to see how that affects the map. Choose a color ramp to your liking.
4.17. Click OK and Done to accept all the changes. Go to the table of contents to see your map layers.
5. Summary
This tutorial demonstrated how to create a new map in ArcGIS online, change the basemap, and search for data and add it to the map. You viewed the metadata for a dataset which described who created the layer and what information is being mapped. You also looked at the attribute table of the layer, which is a spreadsheet containing all the information in the layer which can be visualized. You looked at several ways data of representing data, which can improve the ability of the map to convey your message.
5.1. Save your work! We will add layers to this map later. Click the Save button at the top of the page.