North American Seasonal Precipitation Atlas

Creating Maps

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

Single Year

This tool will create a total precipitation and a standardized precipitation index 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."  The cool or warm season can be selected next to "Season."  Enter the year to map next to "Year to Display."  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 total precipitation and a standardized precipitation index 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 an 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 NiƱo 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."  The cool or warm season can be selected next to "Season."  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.  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 an instrumental or reconstructed total precipitation 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 an 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."  The cool or warm season can be selected next to "Season."  Enter the year to start the correlation next to "Start Year" and enter the year to end the correlation next to "End Year."  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).  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):

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).