Creating Maps

There are three tools for creating maps located under "Maps" in the main menu:

Single Year

This tool will create a PDSI map for a single year from the instrumental or tree-ring reconstructed datasets. Upon selecting this tool, the user will be asked whether they want to create a map from the instrumental or reconstructed dataset.  Select the dataset needed and click "Next." A new page will appear and underneath "Single Year Map," the selected dataset and the time period of data coverage will be shown next to "Dataset."  Enter the year to map next to "Year to Display."  In the case of instrumental data, a single month or a range of months can be selected for mapping.  Selecting a range of months will average (composite) all months from the start month to the end month.  If the range of months crosses into a new year, then end month will be associated with the "Year to Display" (e.g., entering 1940 and selecting to map Dec-Feb, will average Dec 1939-Feb 1940).  See "Map Options," "Error Messages," and "Map Display" below for a discussion about the different options that can be used in the creation of the map, how error messages are generated, and different map files available for download.

Composite

This tool will create a PDSI map that averages a range of years from a start year to an end year or averages years from a list of random years that do not have to be sequential.  Upon selecting this tool, the user will be asked whether they want to create a map from the instrumental or reconstructed dataset.  The user will also be asked how the composite should be computed.  If a range of years from a start year to an end year is need, select "Range of Years."  If a list of random years is needed (e.g., El Niño years), select "List of Years."  Click "Next" and a new page will appear.  Underneath "Composite Map," the selected dataset and the time period of data coverage will be shown next to "Dataset" and how the composite will be computed will be shown next to "How to Compute Composite."  If "Range of Years" was selected, then enter the year to start the composite next to "Start Year" and the year to end of the composite next to "End Year."  If "List of Years" was selected, then enter the years to composite one by one in the box next to "Years to Composite".  Users can also paste the years into the box from another program (e.g., Excel).  Years do not need to be in sequential order.  In the case of instrumental data, a single month or a range of months can be selected for mapping.  Selecting a range of months will average (composite) all months from the start month to the end month.  If the range of months crosses into a new year, then end month will be associated with that new year (e.g., entering 1940 as the start year and selecting to map Dec-Feb, will average Dec 1939-Feb 1940 for the year 1940).  See "Map Options," "Error Messages," and "Map Display" below for a discussion about the different options that can be used in the creation of the map, how error messages are generated, and different map files available for download.

Correlation

This tool will correlate data provided by the user with either the instrumental or reconstructed dataset.  The correlation computed is the Pearson correlation.  Upon selecting this tool, the user will be asked whether they want to create a map from the instrumental or reconstructed dataset.  Select the dataset needed and click "Next."  A new page will appear and underneath "Correlation Map," the selected dataset and the time period of data coverage will be shown next to "Dataset."  Enter the year to start the correlation next to "Start Year" and enter the year to end the correlation next to "End Year."  In the case of instrumental data, a single month or a range of months can be selected for processing next to "Months to Correlate."  The data to correlate with each grid point should be placed in the box next to "Data to Correlate," where the first datum is associated with the start year and the last datum is associated with the end year.  Note that there should be only one datum per line and the entered data should not include the years.  Users can paste the data into the box from another program (e.g., Excel).  In the case of instrumental data, a single month or a range of months can be selected for mapping.  Selecting a range of months will average (composite) all months from the start month to the end month.  If the range of months crosses into a new year, then end month will be associated with that new year (e.g., entering 1940 as the start year and selecting to map Dec-Feb, will average Dec 1939-Feb 1940 for the year 1940).  See "Map Options," "Error Messages," and "Map Display" below for a discussion about the different options that can be used in the creation of the map, how error messages are generated, and different map files available for download.

Map Options

A variety of options are available to customize the map (all have default options pre-selected):

• Title:  Enter a title for the map.  This title can be no more than 50 characters in length, and the box will stop accepting characters once 50 have been entered.  Years are automatically added to the title, so there is no need to include years in the title.
• Significance Test:  This option is available for correlation mapping, and checked by default.  It runs a t-test on the Pearson correlation values, and grid points that do not have a statistically significant correlation will be replaced with missing values (i.e., removed from the map).  The statistical significance is assessed based on the p-value entered by the user.  By default, a p-value of < 0.05 is used, but values from 0.0001 to 0.10 can be entered.  If no significance test is needed, then clear the checkmark by selecting it.
• Latitude and Longitude Region:  The latitude and longitude coordinates can be adjusted to zoom in or zoom out on a particular region.  By default, almost all of the North American Drought Atlas is selected for mapping.  There are two ways to select a specific region.  First, the latitude and longitude coordinates can be entered into the boxes manually.  The top and bottom boxes in the "Latitude Region" represent the northernmost and southernmost latitudes, respectively, while the left and right boxes in the "Longitude Region" represent the westernmost and easternmost longitudes, respectively. Alternatively, there is a Google map under the "Longitude Region" boxes, which can also be used to select a specific region.  Follow the directions to move and resize the box on the included Google map, and as the box is moved and resized, the latitude and longitude coordinates will automatically change in the boxes next to "Latitude Region" and "Longitude Region" above the map.  If needed, press the + or - buttons in the lower right of the Google map to zoom in or zoom out, respectively.  Note the maximum and minimum values associated with each region.  Values above or below those are outside of the data available within the drought atlas and cannot be plotted.
• Latitude / Longitude Grid Lines:  This option, not checked by default, will display the latitude and longitude grid lines on the map as dotted, black lines.
• Scale Range:  Choose the minimum and maximum values for the scale.
• Scale Interval:  Select the scale bar interval.
• Color Bar:  Select a color bar.  Two gray-scaled color bars are also provided for black and white maps.
• Map Shading:  Two options for shading are available.  The default is "Shaded Grid Boxes," which represent the true nature of instrumental and reconstructed data in the form of square boxes.  Selecting "Shaded Contours" will draw interpolated colored contour bands.
• State / Province Boundaries:  "Show Canadian Provinces," "Show US States," and "Show Mexican States" are checked by default.  In order to not plot them, clear the checkmark by selecting it.

Error Messages

Numerical data entered by the user are checked for errors (e.g., verify the start year comes before the end year).  If an error is detected, a red x will be displayed ().  Hovering the mouse pointer over the red x will display the error message (see example below).

Sample Error Message.  In this example, the user-specified year next to "End Year" does not exist in the dataset.  Thus, the error message reminds the user of the years available in the dataset.

Map Display

Once all of the options are set, click on the "Create Map" button to generate the map.  The process could take a minute.  Once the map is created, it will be displayed.  If any data are not available for a particular land area on the map, then a gray color will be displayed.  Use the back button on the browser to go back to the previous webpage if something needs to be changed, and click the "Create Map" button again.  Once satisfied with the map, it can be downloaded in three different formats using the buttons underneath the map.  Click "Download .png" to download the image in png format.  Click "Download .eps" to download the image in eps format, which is good for further editing in programs like Adobe Illustrator.  Click "Download GeoTIFF" to download the data in a tif format that is friendly to GIS programs like ArcGIS (note missing data in the GeoTIFF format are denoted by -999000000).