Learning to Work with Temporal Data in ArcGIS Working with a netCDF File in ArcGIS

Objective NetCDF (network Common Data Form) is a file format for storing multidimensional scientific data (variables) such as temperature, humidity, pressure, wind speed and direction. Each of these variables can be displayed through a dimension (such as time) in ArcGIS by making a layer or table view from the netCDF file.

In this tutorial, you’ll learn how to create a netCDF raster and a table and how to display a specific time step. You will also create a temperature profile at a location and finally perform some analysis with netCDF raster layers.

Locating data and map documents • Data folder: TDinArcGIS\Exercise2\Data • NetCDF file: temperature.nc, Variable: tmin (winter minimum temperature) • Map documents: NetCDFinArcGIS.mxd • Toolbox: AnalysisWithNetCDF.tbx

A: Making a netCDF raster layer In this exercise, you’ll create a netCDF raster layer using the variable tmin. You will change the display by selecting a different time step.

A1: Opening an existing map document 1. Start ArcMap by using the Programs list on your Start menu. 2. If the ArcMap startup dialog box appears, check An existing map, then double‐click Browse for maps. If you previously checked Do not show this dialog again on the ArcMap startup dialog box, open the map document by clicking Open instead. 3. Navigate to the TDinArcGIS\Exercise2\Data folder on your local drive, click NetCDFinArcGIS.mxd, then click Open. Nothing is displayed but it is not empty. A table and a graph are saved in the document for exercises B and C.

A2: Making a netCDF raster layer 1. In the ArcToolbox window, expand the Multidimension Tools toolbox.

2. Right‐click Make NetCDF Raster Layer and click Open or double‐click the tool. 3. Type the name in the Input netCDF File, or alternatively, click the browse button to navigate to the input file temperature.nc in TDinArcGIS\Exercise2\Data.

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4. Click Variable, click the Variable drop‐down arrow, and select a tmin from the list. 5. Click X Dimension, click the X Dimension drop‐down arrow, and select lon from the list. 6. Click Y Dimension, click the Y Dimension drop‐down arrow, and select a lat from the list. 7. Type tmin_Layer in the Output Raster Layer. 8. Click OK.

A netCDF raster layer is added to the table of contents (ToC) displaying the winter minimum temperature of 1875 from the default (first) time slice. Note that the minimum and maximum temperatures are ‐59.0669 and 27.6854 respectively.

A3: Displaying a specific time slice You can display a different slice of a netCDF raster layer, feature layer, or table by changing dimension values on the netCDF property dialog box. 1. Right‐click the netCDF raster layer tmin_Layer in the table of contents and click Properties.

2. Click the NetCDF tab.

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3. Click the value field next to the dimension year. A drop‐down arrow will appear. 4. Click the drop‐down arrow and click 1/1/1940. 5. Click OK on the Layer Properties dialog box.

The netCDF raster layer tmin_Layer is now displaying the winter minimum temperature of 1940. Note that the minimum and maximum temperatures have changed to ‐60.2398 and 28.153 respectively.

A4: Returning to the default time slice You can return to the default time slice by changing the dimension on the layer property dialog or you can use the Select By Dimension tool.

1. Right‐click Select by Dimension and click Open or double‐click the tool. 2. Click Input Layer or Table drop‐down arrow and select tmin_Layer from the list.

3. Click OK.

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The netCDF raster layer tmin_Layer is now displaying the default time step i.e. the winter minimum temperature of 1875. Note that the minimum and maximum temperatures are now back to ‐59.0669 and 27.6854 respectively.

B: Creating a profile at a location In this exercise you will create a temperature profile at a location. You’ll first create a netCDF table with predicted temperature values. A graph is already created for you. You will examine the graph and its properties.

B1: Making a netCDF table view 1. Right‐click Make NetCDF Table View and click open or double‐click the tool. 2. Type the name in the Input netCDF File, or alternatively, click the browse button to navigate to the input file temperature.nc in TDinArcGIS\Exercise2\Data.

3. Click Variables, click the Variables drop‐down arrow, and select tmin from the list. 4. Type temperature_View in the Output Table View. 5. Click Row Dimensions, click the Row Dimensions drop‐down arrow, and select year from the list.

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6. Click Dimension Values, click the Dimension Values drop‐down arrow, and select lon from the list. The dimension will be added to the table. Click the Value drop‐down arrow next to the added dimension and type in ‐111.75. 7. Click Dimension Values, click the Dimension Values drop‐down arrow, and select lat from the list. The dimension will be added to the table. Click the Value drop‐down arrow next to the added dimension and type in 45.75. 8. Click OK. 9. Click on the Source tab in the table of contents.

10. Right‐click the netCDF table view temperature_View in the table of contents and click Open. Examine the fields and values in the Attributes of temperature_View.

In the next step you will display a graph created from a netCDF table view similar to the one you just created.

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B2: Displaying a graph created from a netCDF table 1. Click Tools on the Main Menu, point to Graphs, then click Minimum Winter Temperature.

The graph displays the predicted change in minimum winter temperature from 1875 to 2095 at longitude = ‐111.75 and latitude = 45.75.

2. Right‐click anywhere on the graph, then click properties. Examine graph properties such as Graph type, X field, Y field ctc.

3. Click OK after you are done.

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The graph shows a significant increase (shift) between years 1935 and 1940. In the next exercise we will create a temperature difference raster between these two time slices.

4. Close the graph by clicking the cross at upper right corner. 5. Close the table Attribute of temperature_View. 6. Click on the Display tab in the table of contents.

C: Finding difference between time slices The graph shows a significant increase (shift) between years 1935 and 1940. In this exercise you’ll create a temperature difference raster between these two time slices by using a model.

1. Find and expand the AnalysisWithNetCDF toolbox in ArcToolbox. 2. Right‐click the model FindingDifference, then click Edit.

3. Examine the model. Double‐click on Make NetCDF Raster Layer process and examine parameters. Note that Dimension Values parameter is used to create a raster for a specific time step.

4. Run the model by clicking the run button. Raster layer tmin_diff will be added to the table of contents.

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Move or minimize the model so that you can see the tmin_diff layer in the display.

5. Double‐click on the legend of tmin_diff and select a suitable color ramp from the dropdown list.

6. Click OK.

The differences of temperature between two time steps are significant and vary across the area of interest. Two shifts on the graph also suggest some discrepancies in the time‐series. We now need to go back and check the original (simulated monthly temperature) data and how we processed the data to derive this average minimum winter temperature.

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