Tarsier tutorial: Raster data

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.

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. Click 'Download' in pop-up window that appears
11. Save zip to local machine and extract to folder
12. The folder should now contain an Arc GRID coverage - a bunch of files requiring ArcGIS software to open

= Convert Arc GRID file format to Arc ASCII GRID format

Tarsier can't import native Arc GRID files. But it can import Arc ASCII GRID files that have been exported from Arc format using ArcGIS.

1. Open ArcMap
2. Add Data - your Arc GRID raster
3. Open ArcToolbox
4. Select Conversion Tools --> From Raster --> Raster to ASCII --> Select output file as something like "kauai.asc"

Importing raster data into Tarsier format

1. Open Tarsier
2. Open your ASC file e.g. "kauai.asc"
3. 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