Working with Grids and Images

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Working with Grids and Images
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Working with Grids and Images

• Raster data
• Surfaces grids of pixel picture elements
• Each grid cell has a unique value
• Two types
• Continuous store values with decimal places (eg
  temperature or rainfall)
• Discrete represent data with a discrete value
  (eg land use or vegetation)

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Grids Images Rasters

• Rectilinear arrays of numbers
• Value types
• Integers only countable data or classifications
  (e.g. vegetation codes)
• Floating point typical data from measurements
• Saga used for visualizations, conversion
• GDAL library in Saga includes many formats

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Most Familiar Raster ESRI ASC
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• ESRI Grids
• Header

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Typical HDF Operations

• Obtain grid ( metadata source link)
• If needed, use HDFView to extract ASCII grid
• Convert the ASCII to XYZ or FLT for further work
• Use Saga to make geo-referenced TIF files
• Regular GeoTIF for image use
• Data GeoTIF for WMS use

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Typical ESRI Raster Operations

• Obtain data grid ( metadata link to source )
• Check the format for errors/surprises
• If ASCII Save as DOS ASCII with proper filename
• Import into Saga
• For clipping or blanking, see next slide
• Set file parameters (no data, colors, min/max,
  etc.)
• Export image (.TIF) for general atlas usage
• Export integer or floating point raster
  (different .TIF) for WMS
• Special new exercise available to explain the
  steps
• In windows explorer, fix filename extensions for
  above products

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• Modifying Grids with Saga

• Blank (or mask) a grid
• Create landmask with QUIKMASK (zero values over
  land)
• Run Grid Tools gt Resizing
• Resize mask to desired geometry, using Nearest
  Neighbor method
• Run Modules gt Grid Calculus gt Grid Calculator
• Divide to-be-masked grid by landmask grid
  division by zero results in NO DATA values over
  land
• Subset a grid
• Load grid into Saga
• Export grid as ESRI binary (FLT plus HDR)
• Use FLT2FLT to subset the grid as needed
• Load new gird into Saga for further processing

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File-Naming

• Use lower case letters numbers for files
  folders
• No spaces
• No punctuation except underscore lt_gt within
  names or period lt.gt before extension
• Always include 3-letter filename extension (jpg,
  gif, htm, pdf)
• Limit the total number of characters to 32
  including the extension
• Note There is a 13-character limit for ESRI grid
  names, but we wont consider this until it causes
  problems

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File-Naming

• Source_Parameter_Time_Depth_TechInfo
• Source dataset, collection, archive, etc.
• Parameter simple variable name
• Time Annual, seasonal, monthly, specific
• Use JFM etc. for seasons to avoid confusion
• If specific date, use ISO standard
• Depth Only for ocean samples
• TechInfo Anything else of significance, e.g.
  the unzipped format of zipped files

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Filename Examples

• coads_airtem_amj_500m.asc ESRI grid of air temp
  data for the boreal spring from the COADS data
• modis_chloro_annu_0m_hdf.zip MODIS annual
  average surface chlorophyll data in HDF format,
  zipped
• modis_chloro_20071201_0m_hdf2tif.zip Similar
  data for a specific date, converted from original
  HDF to TIF, zipped