knitr::opts_chunk$set(echo = TRUE)
An AOD scan of the Continental U.S. is captured every 5 minutes of the day and typically yields around 3 to 4 MB of data. This means that for every hour of the day, 12 files are generated. An entire 24-hour day would have close to 300 files in it and be close to a Gigabyte in size. This would include scans taken during nighttime hours which do not contain valid AOD data.
AOD scans are stored by timestamp using Coordinated Universal Time (UTC). It is assumed that a person working with this data is generally working in local time, and will need to have access to the appropriate AOD scan that corresponds with a specific local time in the U.S.
Finally, as meantioned in "Introduction to MazamaSatelliteUtils", there are 2 GOES satellites, GOES-East and GOES-West, which favor different parts of the U.S. with their coverage. "GOES-East Data Quality Spatial Limits" provides a useful overview of the Western limit of coverage for GOES-East. Generally speaking though, when looking at the Western U.S., one should use data from GOES-West.
The MazamaSatelliteUtils package provides easy ways to specify time(s), locations and the GOES satellite of interest.
Time can be specified in a number of ways in MazamaSatelliteUtils, but it is important to keep in mind that the unit of time the package works with is the hour. More granular times can be given as parameters, but the package will truncate them to the hour.
Furthermore, it should be clear that a specific date is also required for the various utilities to work. This combination is referred to as a "datetime" in the documentation. Below are some of examples of how a "datetime" can be specified.
(Note that all examples are written as parameters of a function call)
datetime = "2019-08-01 10:00", # ISO 8601 timezone = "America/Los_Angeles" # local time datetime = "2019080110", # YYYYmmddHH timezone = "America/Los_Angeles" # local time datetime = "2019-08-01 17:00", # ISO 8601 timezone = "UTC" # UTC datetime = "201924417", # Julian date isJulian = TRUE
Two requirements must always be met when specifying datetimes:
The GOES-East and West satellites are easily specified in by using the SatID
parameter. For ease of use and clarity, the package implements GOES-East as "G16",
and GOES-West as "G17". Below are examples of how SatID
is used in a function call.
satID = "G16" # GOES-East satID = "G17" # GOES-West
While it can be useful to work with the entire extents of CONUS when analyzing AOD data, often it is desirable to filter, or limit, the data to a specific geographic extent or location. Depending on the function being used, this can be a region, or a point. (See the specific function documentation to determine which of these it supports.) For extents, the package allows the user to specify a region of interest in several ways. For clarity and consistency the parameter used to specify extents is named "bbox".
# Ex 1. bbox as a list of coordinates in lonLo, lonHi, latLo, latHi order function( bbox = c(-124.56624, -116.46350, 41.99179, 46.29203) ) # Ex 2. bbox obtained from spatial object oregon <- subset(USCensusStates, stateCode == "OR") bbox_oregon <- sp::bbox(oregon) function( bbox = bbox_oregon )
NOTE: spatial object example assumes that "USCensusStates" is available from the
MazamaSpatialUtils
package.
As mentioned above, working with AOD data can lead to having a large number of
files on disk very quickly. MazamaSatelliteUtils requires that a data directory
be specified for storing these files. This is done using the setSatelliteDataDir()
function.
"Example Workflow" gives an example of how to use these concepts to do a simple analysis.
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