LAS Help & FAQs

Table of Contents:

General Help

Known Issues:

How To:

General Help:

LAS holds datasets that span up to 30 years of daily data.  However, trying to do analysis over the whole time range is not feasible.  Depending on the number of granules and the resolution of the data, users can reasonably expect a range of 1 year (high resolution data) to 10 years (low resolution data) to complete analysis.  The time it takes and the exact amount of data varies with traffic on the LAS site.
 
LAS has many small “?” buttons – pressing them will bring up information either about the dataset or the feature, depending on the context.  


Known Issues:

Animate Error:

Animation gives an error with large datasets when requesting more than ~3 granules.  LAS support is looking into this. 

Example:




 

Later versions of LAS color everything as purple, or the lowest value on the color scale.

If this error is encountered, the animate tab needs to be closed, and Animate restarted from LAS.

 

Currently, the workaround is to request a smaller number of granules, and then after the request completes successfully, to add onto the request a day at a time.  This gets the max range to ~9 granules.


Large Dataset Error ("Remote Server Error")

"A remote server was unable to deliver the data LAS needs to make your product." 

This error occurs with very large datasets, or high volume datasets.  LAS support is aware of the issue and looking into it.  The current work-around is to request a smaller range of time.


 


Exporting NetCDF Files:

Exporting (saving out) as netCDF does not save the Offset parameter, giving incorrect values when opened.  We are working to resolve this issue.  As a workaround, the Offset can be found in the header of the original file, and can be added to the data manually.


Map Scale not found:
LAS saves all request results.  The upside is that if the same request is made again, LAS returns the results immediately.  The downside is that periodically the disk completely fills, and this error is returned.  We are aware of the issue and working on a long term automated solution.

Currently, LAS is not configured to automatically apply quality flags to data sets.  Nor is LAS capable of constructing data sets on the fly from other data sets.  We are working at this time to incorporate quality flags into data sets as well as other constructed data sets. 

HDF5 data set files and Level 3 data sets are currently not being read correctly into LAS, as LAS is designed for netCDF files.  We are working on ways to side-step this issue, and appreciate your patience.
 
For the moment, these files can be manually converted using the "Set Plot Option" -> "Evaluate Expression".  Please see "How to get MODIS to display units and screen fill values".


How To:

How to have LAS update the plot automatically:

Check the box on the Update Plot button.


How to change the color range:

Go to Set Plot Options, and to the Color Fill field.

Enter the desired range, in the format (min, max, step). 

Press okay and update the plot.
 

Visual Example:

Click on the Set Plot Options button.



In Color Fill, type in the range  desired, as such: (min, max, step)



In this example, we first choose (272,300,1)



Gives:


 

This has lots of white, areas that must be out of the range.  To reserve the top and bottom colors for all the values out of the color range, use (lower bound)(min, max, step)(upper bound).

For example: (260)(272,300,1)(310)



Gives:



Refining the range: (260)(270,305,1)(210)



Gives:


 


In the 2-4 panels, next to the “i” button, there is a “Settings” button.  Click this to open up a new set of tabs; the available tabs are type of plot, data sets, and plot options.  Click on data sets.




Then select the date desired, and click “Update Plots in the upper left main suite of buttons.  The lat/lon coordinates may need to be adjusted to match the first granule's coordinates.




 

 
Go to Set Plot Options, and to the Color Fill field.
Enter the desired range, in the format (min, max, step). 
Press okay and update the plot.
 
Visual Example:

 

Click on the Set Plot Options button.



In Color Fill, type in the range  desired, as such: (min, max, step)



In this example, we first choose (272,300,1)



Gives:



This has lots of white, areas that must be out of the range.  To reserve the top and bottom colors for all the values out of the color range, use (lower bound)(min, max, step)(upper bound).
 
For example: (260)(272,300,1)(310)



Gives:



Refining the range: (260)(270,305,1)(210)



Gives:



 
LAS creates the plot using the units given in the dataset.  To change units displayed on the plot, go to Set Plot Options, to the Equations field.
Type in the equation to be used, click okay, and update the plot.  
 
Visual Example:


Set the Equation to convert Kelvin to Celsius in the Evaluate Field:  $ -273.15





Gives:



Converting from Kelvins to Fahrenheit:  1.8 * $ - 1.8*273 +32
 
(The equation evaluation is finicky on what it will accept; here, we distributed 1.8 in order to follow the syntax.)



Gives:



 
This is done with a combination of unit conversion and color bar manipulation.
 
The MODIS scale and offset have been applied as a way for users to visualize the data for the correct temperature range and to apply an "approximate" filter mask. However it is not intended to replace the application of the MODIS quality flags. For a rigorous cloud filtering of the data one must still download both l3m_data and l3m_qual  and apply the MODIS quality flag of 0 in l3_qual to l3m_data to extract the highest quality SSTs.
 
Here is how MODIS looks when first displayed - notice the lack of units and the strange range of the color bar.



First, use the Set Plot Options -> Evaluate Expression to change the unitless values to known units.  The equation is:  0.000717185 * $ -2, as the scale factor (slope) is 0.000717185, and the intercept is -2. Enter the equation and click "update plot".





Now to filter out the "Fill values".  Open Set Plot Options, and in the Color Fill Level, enter (0,32,2), click OK, and then Update Plot. This sets the minimum to 0 C, the maximum to 32 C, and has a color step of 2 degrees.





 
Where to find the metadata information button when a data set is loaded.


Clicking on the “?” opens a new browser window with metadata and links for the dataset & variables: 



Clicking on “See the URLs” gives this:



These links go to the OPeNDAP URLs.  This is great for direct downloading, but also for viewing information about the variables in full – which is what we’re after in this instance.  Clicking on the link for the l3m_data goes to OPeNDAP:



Scroll down to the variable being plotted in LAS – l3m_data in this case:


Here we see the scaling equation is linear, with the Slope = 0.000717185, the Intercept = -2.0. 
 
From here, enter the scaling equation to LAS's evaluate equation setting.