GridComp faq

From Earth Science Information Partners (ESIP)


Choosing a Dataset

There are currently 4 default datasets immediately available, but any dataset present in the datafed wiki RSS catalog may be brought into the application.

To choose a dataset from the catalog, select "Choose new dataset" from either map's dataset drop down menu. GridComp_new_dataset.PNG The Grid datasets from the datafed catalog will pop up in a new window.

From here, you may search for values in a number of fields, view the dataset's metadata, or select the dataset to bring it into the Grid Comparison Tool application. The chosen dataset will be available for both grids, and you may select as many at a time as you wish. For our comparison, let's select the PMFINE parameter from both CMAQ_EPA and CMAQ_CENRAP.


Choosing a Time

Once you have selected a dataset, you will notice the time range available for that dataset and the frequency of data available below the options. GridComp_time_print.png

You have the option to view a single instance of this dataset, or combine data from multiple time periods. If you wish to view a single instance, set the year, month, day, and hour to an available time. NOTE: Daily, Monthly, and Yearly datasets have data recorded at the beginning of each period. For example, ABBI is a monthly dataset, thus data is only available on the first day and first hour of each month. Make sure hour is set to 0 and day is set to 1 for monthly datasets.

If you wish to select a range of dates, change the Time selection box to "Time Range". You will notice the Year,Month,Day,and Hour selection boxes are now disabled. To select a time range, use the Time filters that have appeared to the right. Use control+click to select any number of years and months available to the dataset, followed by the operation you wish to perform to aggregate the data.


The CMAQ data only has 1 time available, so set the time instance to July 18, 2002.


Preview a Dataset

Once you have selected a time and dataset, you may set a scale minimum and maximum for the window. There are 6 colors displayed in the map, ranging from blue at the scale min to red at the scale max. Note: This scale is only for this map. You may use exponents in the form "5e05" or "2e-09" or negative numbers.

PMFINE data for CMAQ runs max out near 50, so set the scale max to 50.

Preview the data by selecting "Update Map".


Performing a Calculation

Now that you have two datasets and a time selected, you may compare these datasets with the options to the right of the big map. Choose an expression, a scale, and a resolution for the resulting data. Custom expressions are allowed by choosing "Custom" from the expression drop down box. Type in your custom expression in the text input that has become available. Use "a" to refer to your left grid and "b" to refer to your right grid dataset. For example: (3*a-2*b)/4.

For CMAQ data, we wish to find the difference, so just set the expression select to a-b (or custom: a-b), and then hit Calculate.


Navigating the Maps

Grid a and b follow movements of the large calculated map. To move any map, click and hold somewhere inside the calculated map and drag around to pan. You may also use the arrow keys near the top left of the map to step along in the cardinal directions.

To zoom in or out, you may use the mouse wheel (mouse wheel up is zoom in, down is zoom out) when hovering over the large map. You may also use the "+" and "-" that are next to the globe near the top left as well as a double-click to zoom in. To zoom in to a specific area, you may select the zoombox tool and drag out a rectangle to be viewed. OpenLayers has a set number of zooms, so you may not get exactly what you want, but it should be close. To change back to click and pan mode, click the hand underneath the zoombox tool.

GridComp_toolbar.PNG GridComp_CMAQ_zoom.PNG

Saving Your Work

All the options and datasets you have chosen may be saved in the NEISGEI Portal for yourself or others to view and work with later. To do this, click the "Save Session" button just above grid b. You will be asked for your name (Anonymous is fine), a title for the session, and a short description of the data. If everything goes well, you will see an alert box telling you the session has been saved. To view the saved sessions for this tool, visit this link.

At times, the portal may lag behind recently saved sessions, so in case you do not immediately see your new session, click the "Refresh Data" link near the top of the portlet. NOTE: A user account to the portal is required for this to work. You may sign up for an account here.

Selecting your session inside the portal, you will see a display of the options you have chosen. You may leave a comment about this session by clicking "Post Reply" (Again, user account to NEISGEI Portal required).

You can revisit your work at any time by clicking the "View saved analysis" link.

Exporting Data

All of the data viewable in the Grid Comparison Tool is available in a variety of other formats. To view a dataset or calculation in another way, select your desired format from the dropdown list next to "Export as" located by any of the three maps. KML, CSV, NetCDF, PNG, Jpeg, GIF, and 8-bit GeoTIFF are available.

Once you have selected a format, click "Export as". A new tab or window will open soon that should give you an option to save or view the data.


North American Borders

A more detailed map of North America is available to be placed on top of your data if you wish. To do so, click the blue "+" that is sticking out from the right of each map. You will notice the World Borders base layer, with your dataset as an overlay and "North American Borders" unchecked. Simply place a check in the box to the left of North American Borders, and the map should quickly display state and province borders for North America.

GridComp_NAselect.PNG layer selector

GridComp_CMAQ_NAborder.PNG NAmerica borders displayed

Syncing data and tips

If you find yourself wanting both datsets to be the same time and/or scale, you may choose a Syncing option from between Grid a and b. Choosing the Time sync option and changing a date in grid A will set the same date in grid B and vice-versa. Choosing the Scale option will keep the scales between grid A and B the same.

You may also use the Previous/Next Day/Hour buttons to step through data. Each click will update the day or hour and refresh the map. If you have the time sync option selected, it will do the same for the opposite map.

If you wish to view the map on a larger scale, right click on the desired map and select "View Image". Note: North American boundaries must be deselected for this. You should see your same map alone in the window now with a long and complicated URL as the web address. At the very end of this address are WIDTH and HEIGHT parameters. You may set these to whatever width and height you would like to view your map up close.


The comparison tool is pulling data from an RSS feed and feeding it into the DataFed system. Occasionally, there are some errors.

The most common error is from selecting a date and time for a dataset that has no data. When this happens, you will see either a blank map (no data returned) or a pink error screen.


When this happens, check your date and make sure it is inside the allowed range for that dataset.


You may contact Ed Fialkowski at or Stefan Falke at with any questions.