ArcView Lesson 2
Raster Graphics and New Themes
April 12, 2011
Rasters are digital elements organized in a matrix (rectangular array of numbers) in a format that the computer converts into an image. Pixels are used to define how many digital elements a screen can show. Digital cameras confuse the situation by calling their image size pixels - a pixel is an element of your monitor screen or television.
Please open this page in Microsoft Explorer so downloading the Zipped files needed is simplified. In Explorer, right click link, 'save target as' to your thumb drive
Orthophotos and topographic maps:
Today's files include topographic maps and aerial photos that have been scanned and registered. The process of geo-registration links location information with the image file, creating a geotif or a SID file.
Begin a new project in ArcView. Before adding themes to your view, locate the Extensions menu under the File menu. Check the Mr.Sid support box. This enables the software that allows ArcView to use .SID files (a greatly compressed image file - it takes 10 CD-ROMs to hold my topographic map images of Honduras in geotif format, one CD holds the entire set of maps as .SID files).
Projections and datum issues: Raster graphics are frequently projected in Universal Transverse Mercator (UTM) using meters or kilometers as units. ArcView cannot distort image themes so that they can be seen in other projections. In UTM, the y coordinate is usually measured north from the equator, the x coordinate east of a central meridian, with 500 kilometers 'false easting' added so that all x values are positive. There are 30 UTM zones in each (north and south) hemisphere, each zone 6 degrees wide, bounded by meridians 3 degrees east and west of a central meridian, each an integral multiple of 6. Toledo lies in zone 17N, bounded by meridians 81 West and 87 West, with central meridian of 84 W. Zone 16 lies to our west, zone 18 to our east.
Datum refers to the earth model and origin of a UTM (or other coordinate) system. USGS topographic maps printed before the mid-1980s include NAD27 (North American Datum specified in 1927) reference tics. Except for one Toledo topographic map, today's raster graphics are registered in NAD83. The Toledo map that does not reside in the NAD83 folder is in NAD27. You might take note of the fact that, although these maps are identical in every other detail, they do not overlie one another perfectly because datums differ.
Today's maps include edges - they have not been trimmed and edge matched. I have other GIS software that can do that.
After unzipping the downloaded folder, launch ArcView and, under the View menu, 'add theme' all of the features. These are image features, not vector (shapefiles). Under View, set Properties - map units and distance units as meters.
Demo (Dr.D.): NAD27 vs NAD83 topographic map. After demo, delete NAD27 theme.
The aerial photographs are orthophotos. Standard aerial photographs are taken in line as a series of photos that view ground features from changing angles so that, with the help of special viewers, you can see stereo and even measure heights of buildings, trees and mountains (the latter not applicable for Toledo). Standard aerial photos are not true to the map - features increase in location error as you move from the center to the edge of a standard aerial photo. Orthophotos have been processed to see things from directly overhead. You cannot see stereo even with the special viewers but these photos are true to the map.
Photos to the west of our study area (Lucas County) are referenced for both UTM zone 16 and UTM zone 17. ArcView cannot discriminate UTM zone or datum, the program will allow you to open themes of different datums and units without ringing a warning bell. One file I failed to include in the folderr was metadata - a file specifying the units, datum and other information about these raster graphics.
You will use these resources to answer 2 questions:
1. How many square meters of Main Campus parking lot space are documented on these aerial photographs?
2. How long is the drive from a parking lot near Bowman-Oddy Labs on the Bancroft Campus to the Lake Erie Research Center?
Detailed colored aerial photos - right click, save as to thumb drive, unzip. These aerial photos are more detailed and more recent. The entire state of Ohio is available as aerial photos of this quality but just Lucas County requires over a gigabyte of storage space. I have trimmed these to reduce file size while retaining resolution.
Under the View menu, select New Theme. You need to tell ArcView if your theme is points, lines or polygons, where your shapefile will reside and its name. For parking lots, select polygon theme - you will map them with the polygon tool. After you draw the first polygon, open the theme table. Under the Edit menu, add a field. Call the new field area - area is a number (not a string or logical). Use the Edit (I with arrow, middle of second - or lower - toolbar) button, then click the area spot on the table and type in the value. Note that a polygon perimeter and area are shown when the polygon is selected in the View. Then create another New Theme, a line theme, adding major streets to your view. (Note to prof: demo both line theme and polygon theme, and, vertex editor).
You probably know more about parking areas on campus than I do (I live within walking distance). Once you have all parking lots included in your new theme, select all (you can do this in the theme table, they remain selected when you return to the view) and read the area of all selected features.
The Lake Erie Center was constructed after one set of these photos and maps were published (a caveat in using image themes, they are always out of date, some more so than others). It is located at 300320 East, 4618000 North. Move your cursor until its coordinate matches this location. Create a new point theme and place a dot at the Lake Erie Center. Place a second dot in a parking lot near Bowman-Oddy Labs.
Measure the 'as the crow flies' distance between these 2 dots. Now create a new line theme. Navigate the roads through Oregon and East Toledo, cross the bridge of your choice (assume all open - another bad assumption) and find your way to the parking spot (one more bad assumption). I begin a line theme with the line graphic, then use the vertex editor to continue the line after panning the view.
Prepare 2 layouts. One layout shows parking lots on the UT Bancroft Campus (do not include Scott Park or the Medical Campus). Export as a .jpg. Include as labels on the layout your name and the total parking lot area (multiply ramp areas by 3 to get approximate total parking area).
Now do the same for the drive from the LEC to Bowman-Oddy. Include as labels your name and the total distance driven by a graduate student rushing to Friday seminar from the LEC.
Do not display the topo maps or aerial photos as part of your .jpg, these maps are too busy (too much unnecessary - for these problems - information, information that gets in the way).
Next week: aftershock analysis using ArcView queries to isolate the data.