Difference between revisions of "Tarsier tutorial: Raster data"

Revision as of 20:12, 11 February 2009

What are raster data?

Raster data are gridded data such as images and gridded maps of landscape properties such as elevation. The grid is typically two-dimensional, with the dimensions representing coordinates such as latitutde and longitude.

Raster data and vector data are the most fundamental types of data used to represent landscapes.

Examples of raster data include:

• Maps of elevation (digital elevation models, DEMs)
• Maps of bathymetry
• Satellite images
• Aereal photos
• Maps of land cover (also often represented as vector data)

Obtaining raster data

Raster data are widely available on the web.

Useful Data Sources

Example: Topography of Kauai

Obtain a digital elevation model for a portion of the USA - Kauai

1. Visit National Elevation Dataset (NED): http://ned.usgs.gov/
2. Navigate to Seamless Data Distribution System (SDDS): http://seamless.usgs.gov/
3. Select USA
4. Zoom to Hawaiian Islands
5. Zoom to Kauai - the northernmost island
6. Click 'Download' on far right
7. Select 'SRTM Finished 3 arc sec' i.e. the coarest data available, since we just want a quick look
8. Select 'Download' tool on far left
9. Use tool to define the smallest rectangle that includes the entire island
10. A pop-up window appears. Before clicking 'Download' consider which file format you want:
    1. ArcMap Grid File Format - native ESRI ArcGIS format - can only be ready by ESRI products (pretty much)
    2. GeoTIFF File Format
    3. BIL File Format - Band-Interleaved binary format
    4. Grid Float File Format - This appears to be like the common Arc ASCII Grid format, but binary and in two files: 'flt' and 'hdr' (see FLT). It wouldn't be hard to write a Tarsier importer for this format.
11. Which one should I choose?
    1. ArcMap? Works fine, but its not ideal because you need to use ArcGIS software to convert it to ASC format. If you select ArcMap Grid, and you want to import to Tarsier, then you'll need to Convert Arc Grid file format to Arc ASCII Grid format
    2. GeoTIFF? These work in Tarsier, but apparently not in the 32-bit version that USGS is using.
    3. BIL? ...maybe
    4. Grid Float? ...need to write an importer
12. I'm in a hurry, can you just post the files here? Yep:
    1. Kauai 3sec ArcGrid.zip
       1. Converted to ASC format importable directly to Tarsier: TarsierRasterKauaiASC.zip
    2. Kauai 3sec GeoTIFF.zip
    3. Kauai 3sec BIL.zip
    4. Kauai 3sec GridFloat.zip

1. To select a different format to ArcMap Grid, click 'Modify data request'.
2. One the format is chosen, click 'Download'
3. Save zip to local machine and extract to folder

Importing raster data into Tarsier format

1. Open Tarsier, select File-->Open and open the file.
2. Notes for specific formats:
3. ASC
   1. Open your ASC file e.g. "kauai.asc"
4. Once opened in Tarsier, save in Tarsier format. Select "Save As" and save as kauai.tra and kauai.trv
   1. Warning: there's an idiosyncrasy to saving files in Tarsier

Viewing raster data

1. Open a the TRA file or the TRV
   1. The TRA is just the raster data, and the the viewer for the raster data. So when you open a TRA, you get default viewer. If you change the view settings (colors, zoom etc), you can save these in a TV and re-open them later. Each TV is associated with a TRA (and also a second TRA that can be used as a mask, but this is almost never done anymore).
2. Zoom in and out:
   1. ...
3. Change the color scheme:
   1. ...
4. Save an image of your raster
   1. ...
5. Etc.:
   1. ...

Manipulating raster data

1. Algebra etc.
   1. ...

Links

• Tarsier
• Tarsier Tutorials