In this lesson, you'll begin to analyze flooding in Venice. Known to locals as acqua alta, flooding in Venice is a tale of two elevations: the low elevation of the city and the high elevation of the lagoon during extreme tide levels. In the previous lesson, you worked with 2D data, but elevation adds a new dimension: height.
Your ultimate goal is to determine how much of Venice is affected at an exceptional tide, defined as 1.4 meters above sea level. To communicate your results in a meaningful way, you need to be able to visualize your 3D data. In this lesson, you'll add a layer of elevation to your map. Then, you'll convert your data into 3D to better understand Venice's elevation—or lack thereof.
Add and explore raster data
Previously, you worked with feature data: data displayed as discrete objects, or features. While feature data is great for depicting structures, canals, or landmarks, it is not the best way to depict elevation over a continuous surface. For that, you'll use a different type of data, called raster data.
Raster data is composed of many pixels, each with its own value. Although it looks different from feature data, you add it to the map in the same way.
- If necessary, open the Venice Acqua Alta project in ArcGIS Pro.
- On the Map tab, in the Layer group, click the Add Data button.
- In the Add Data window, under Portal, click All Portal.
- In the search box, type Venice_Elevation_Data owner:Learn_ArcGIS and press Enter.
- Double-click Venice_Elevation_Data to add it to the map.
The package is a somewhat sizeable 150 MB and may take several minutes to download, depending on your network connection.
The package contains two layers: Venice 1m and Venice Ground Surface. Leave Venice Ground Surface turned off—you'll use it later.
- In the Contents pane, uncheck the boxes next to Landmarks, Canals, and Structures, leaving only Venice 1m and the basemap visible.
Unlike feature layers, which have shape geometry, raster layers use pixel matrixes in which each pixel stores its own value. The name of the layer, Venice 1m, refers to its resolution: the size of its pixels. The 1m means that each pixel represents an area of one square meter.
- In the Contents pane, click the arrow next to Venice 1m to view its symbology.
Instead of a single symbol, this layer has a color scheme for different values. The values represent elevation in meters. Venice's elevation ranges from just below sea level (black) to about eleven meters above sea level (white)—very flat terrain!
- On the Map tab, in the Navigate group, click Explore. Click anywhere on the raster to open a pop-up.
The pop-up has two values: Stretched Value, which determines symbology, and Pixel Value, which indicates the actual value of a pixel. In the above image, the pixel has an elevation of about 0.88 meters above sea level.
- Close the pop-up.
From examining the map, you may conclude that most of Venice lies only about 1 meter above sea level, with the eastern and western ends of the city at a somewhat higher elevation. Such a low-lying environment leaves Venice susceptible to flooding. To better understand Venice's extremely flat terrain, you'll visualize it in 3D.
Convert a map to a scene
Traditionally, a map displays data in 2D. A scene is a map that displays data in 3D. By default, ArcGIS Pro will convert a map to a global scene, which depicts the entire world as a spherical globe. Since your area of interest is Venice, not the entire globe, you'll alter the settings so the map converts to a local scene instead.
- Click the Project tab.
- In the blue pane, click Options.
- In the Options window, under Application, click Map and Scene.
- For Scene, choose Local.
When you convert your map to a scene, you'll now focus on Venice rather than the entire world at a global scale.
- Click OK. In the blue pane, click the back arrow to return to your map.
- On the View tab, in the View group, click Convert.
Your map converts to 3D, creating a new pane called Map_3D. You can return to your 2D map at any time by clicking the Map tab.
- In the Contents pane, uncheck Venice 1m to turn it off. Check Structures, Landmarks, and Canals to turn them on.
In scenes, layers are designated as either 3D or 2D. Currently, your layers are 2D layers, which is part of the reason why they are still flat. You'll change this later in the lesson.
- Hold down the scroll wheel or the V key and drag the pointer to tilt and rotate the scene. Pan and zoom the same way you would in a 2D map. You can also zoom by right-clicking.
The flatness of Venice's terrain contrasts with the hills in the distance. Since your raster layers do not extend past Venice, where did those hills come from? By default, scenes use a map of elevation data, called an elevation surface, to determine the ground's elevation. This default surface spans the entire world, although at a low resolution (meaning low detail).
- Return to the Venice bookmark.
When you converted your map to a scene, you also converted your map's 2D bookmarks to 3D bookmarks. Both sets of bookmarks are currently identical, so you can use either.
Add an elevation surface
For visualization purposes, the default ground surface accurately depicts Venice as incredibly flat. However, its low resolution makes it inadequate for detailed analysis. To prepare for the next lesson, you'll add one of your high resolution raster layers as a new elevation surface.
- In the Contents pane, check the Venice Ground Surface layer to turn it on.
The Venice Ground Surface layer has similar elevation data to the Venice 1m layer but includes sea level elevation data for some of the surrounding lagoon. The extra data provides context to Venice's elevation and helps set the scene. You'll set this layer as your ground elevation.
- On the Map tab, in the Layer group, click the Add Preset button and choose Ground.
The Add Data window opens. Normally, layers used in your project can be accessed from the project's associated databases and folders. Since you downloaded the Venice Ground Surface layer and added it directly to your map, you cannot access it from your project. Instead, you'll navigate to its location on your computer.
- In the left pane, under Computer, click Documents.
Layer packages are saved to your ArcGIS folder when you download them. The ArcGIS folder is automatically created in your Documents folder when you use an ArcGIS application on your computer. It serves as the default output location for anything you make using an ArcGIS application and is also where your project is saved.
- Browse to the Venice Ground Surface layer using the following path:
- From the Documents folder, open the ArcGIS folder.
- Open the Packages folder.
- Open the folder that begins with Venice_Elevation_Data and ends with an ID string.
- Open the p1 folder.
- Open the raster_data.gdb geodatabase.
The geodatabase contains two files: venice1m (the Venice 1m layer) and venicesource (the Venice Ground Surface layer).
- Click venicesource and click OK.
The Map Properties: Map_3D window opens.
- Click the arrow next to Elevation sources to view the current ground elevation surfaces.
There are two elevation surfaces. The first is the one you added. The second is the default surface.
- Click OK.
The new elevation source is set as the ground in the area around Venice. The default elevation values are still used in the area outside Venice, so you can still see the hills in the background.
- In the Contents pane, uncheck Venice Ground Surface to turn it off.
- Pan, zoom, and tilt to navigate the scene and better view the new ground elevation.
You may have to zoom very close to see the shifts in elevation—Venice is incredibly flat and low-lying! The dramatic lack of elevation puts into perspective how serious flooding can be in Venice.
Display the Landmarks layer in 3D
As you see when you tilt the scene, the Landmarks layer is displayed as a 2D layer, with its push pin symbol flat on the ground. While this layer does not have elevation data, you can give depth to the push pin symbol by changing how the layer is displayed.
- In the Contents pane, locate the Landmarks layer.
The Landmarks layer is part of a group called 2D Layers.
- Click and drag the Landmarks layer from the 2D Layers group to the 3D Layers group.
The push pins appear in 3D, standing upright as you rotate and pan the map.
Extrude the Structures layer
Another layer is flat and shouldn't be: the Structures layer. Unlike the Landmarks layer, the Structures layer has height data in its attributes. To display the layer in 3D, you'll use a command called extrusion, which displays features in 3D by using a constant or an attribute as the z-value.
- In the Contents pane, click and drag the Structures layer from the 2D Layers group to the 3D Layers group, placing it under the Landmarks layer.
The appearance of the Structures layer changes slightly, but it's still flat. To extrude the features, you'll use an attribute to determine each feature's z-value.
- In the Contents pane, right-click Structures and choose Attribute Table.
The table has five fields, one of which is Height. You'll extrude the Structures layer with the values in this field.
- Close the attribute table.
- On the Appearance tab, in the Extrusion group, click the Type button and choose Max Height.
The Appearance tab is contextual, meaning it only displays under certain circumstances. This tab in particular only appears if a layer is selected in the Contents pane. Make sure the Structures layer is selected before proceeding.
- For Field, choose Height. Leave the Unit parameter unchanged.
The features are extruded, meaning they are given a height value based on the selected field. They now appear 3D on the map.
- Save the project.
In this lesson, you converted your 2D map into a scene. Then, you adjusted elevation and rendering settings to display your data more effectively. From your observations, Venice is incredibly low-lying and flat, placing it in danger of even small increases in water level.
In the next lesson, you'll use analytical tools to determine how much of Venice is flooded at a certain tide level and make conclusions about the difficulties faced by the city.