Lab 2

LAB 2 Notes, Assignment, & Exercise

Discussion of:

Data capture techniques (emphasis on digitizing and GPS)

Map scale

Coordinate systems

Projections

GPS

Range of precision. That is, how tightly grouped readings are of the same coordinate. Range of accuracy. That is, how close the GPS reading is to the true position of the feature. There is always some error in the reading.

Precision, Bias, and Accuracy
For best reception, need 4 satellites, 3 to triangulate position and 1 as a time reference.
2 frequencies – L1 (civilian) and L2 (military).
Post processing is necessary to correct the error introduced.
When capturing line or polygon features, determine how many points you want to capture to adequately describe the feature of interest.

Digitizing

Data are in a vector format and usually come from a general or thematic map. Data captured are only as good as the source data and are usually worse. Error comes from deformed media, precision and accuracy (or lack there off) of the digitizing instruments, and operator error.

Scanning

Data are in a raster format. Often vector data are captured by scanning. An algorithm computes the placement of features. This introduces error. Data are captured as a composite image therefore features of different themes cannot be separated. Scanning also captures stray marks and unwanted data from media.

Remote Sensing

Data are captured in a raster format. Sensing techniques include active and passive sensing. In either case, radiation emitted or reflected from an object is captured as a reflectance value. All features have a unique signature, or reflectance value. These values are classified so similar signatures represent specific features. Capturing data via remote sensing is very expensive and highly technical.

Minimum Mapping Units/Resolution

Vector format – distance between vertices
Raster format – cell size

Map Scale

The map scale describes the relationship between map units and real-world units. Scales can be described 3 ways:

Ratio – 1:1000000

Verbal – One inch equals a mile

Bar – |------|-------------|

0 25 100

The larger the scale, the larger the feature appears. The smaller the scale, the smaller the features appear. The minimum mapping unit at which data is captured ultimately affects the level of detail at which features can be displayed.

Coordinate systems, projections, and transformations

*Public Land Survey System – PLSS

Township, Range, Section

Typically, coordinates are represented by a Northing (y), an Easting (x), and elevation (z). Think of the Cartesian coordinate system from geometry. Coordinate systems include Latitude/Longitude, decimal degrees, and UTM.

Latitude & Longitude

Latitude/Longitude are measured in degrees, minutes, and seconds (or DMS). The coordinates for Talkeetna, AK are 62^o 18" 00’, -150^o 08" 30’.

Decimal degrees

Decimal degrees (or DD) are the decimal equivalent of DMS. The conversion equation is DD = Degrees + minutes/60 + seconds/3600. The coordinates for Talkeetna, AK are 62.3, -150.1383.

UTM

UTM coordinates are stored as meters from a designated control point.

Stateplane

The Municipality of Anchorage uses this coordinate system. Stateplane uses the NAD27 datum, the units for this coordinate systems are feet, and Anchorage is in Zone 4 (or 6176 for ArcInfo purposes).

Projections

Spatial information is stored in a coordinate system, defined by the map projection. Projections attempt to apply a 2-dimensional coordinate system to a 3-dimensional surface. We know it is impossible to flatten a sphere without causing tears and distortion. Thus, map projections are mathematical representations of projecting the coordinates of the earth (in 3D) onto a sheet of paper (in 2D). Each projection carries some type of distortion. These types of distortion are:

Shape
Area
Distance
Direction

Which type of distortion you accept/reject is based on the application of the map you are creating. In this sense, you have to assess which projection is best for a specific application. Therefore, you would choose a projection that least distorts whatever it is you are trying to measure.

Mercator projection

This is the projection often seen in airline magazines. It is a cylindrical projection, and it retains true direction. That is, a straight line on the map (a rhumb line) describes a true direction. This is a good projection for navigation. Distortion increases with latitude; therefore Alaska is appears larger than it really is. UTM is the coordinate system for the Mercator projection.

Gnomonic

This projection is also used for navigation. It is an azimuthal projection. A straight line on the map is a great circle route, or the shortest distance between 2 points.

Albers Equal Area

This is the projection we will be using to map Alaska. It is a conic projection and is good for showing areas the tend east-west. It is often used for showing the United States. As the name states, areas on the map are proportional to those on the Earth.

Assignment(s):

GPS 3 features on campus that you will use in your suitability analysis.
Create a directory in your workspace titled Lab2.
Create an ArcInfo coverage or ArcView theme of the features you captured, and save them in your Lab2 directory.
Populate the FAT(s) with the attributes you will need in your analysis.

Lab Exercise

User accounts on \\nanuq\nebula. Same user name and password as last semester, or that you use in Jason’s class. If you are in SC 314 and SC 435, please make a folder titled "SC_435" and put the work for Land Use Planning in there.

Data is available on Kink_d. Map that drive and look around for what you may want to use.

Municipality of Anchorage data

Great Land Trust data (areas of habitat for many animal species)

Digital Elevation Models (DEMs)

Department of Natural Resources (statewide data)

University of Alaska, Anchorage (not quite here yet, hopefully soon!)

Also, consult the GIS Lab website for data available on CD. (http://www.alaskapacific.edu/gis/index.htm)

Capture GPS locations in Latitude, Longitude. Record on paper the x,y coordinates for the feature(s) in which you are interested. Be sure to record them in the order you captured them! Order matters for purposes of topology. Also, think about what is the x coordinate and what is the y coordinate. Make sure you have them in the correct sequence!

Convert to decimal degrees.

Decimal degrees (or DD) are the decimal equivalent of Degrees Minutes Seconds (DMS). The conversion equation is DD = Degrees + minutes/60 + seconds/3600. The DMS coordinates for Talkeetna, AK are 62^o 18" 00’, -150^o 08" 30’, the DD coordinates are 62.3, -150.1383. Carry out to at least 4 places to the right of the decimal point.

Remember, you are transforming from DMS to DD using the above conversion equation. Your data will be in the Geographic projection. You may have to (hint, hint) reproject your data from Geographic to the projection in which the other data you want to use are projected.

No matter what software you use, open HELP and read about what you are doing so you have more information than what follows!!!

Inputting point data in ArcView.

Open a new table and add the fields (Edit, Add Field) you are interested having in your table. I suggest at least ID, X Coordinate, and Y Coordinate. Be sure you set the data type and number of decimal places. Add new records to the table (Edit, Add Record). Once you have a blank record click on the Edit icon on the Tool Bar. Add data in the appropriate spot. Stop editing and save your edits (Table, Stop Editing). Now you need to build a theme out of this data. To do so, make the View Window active and go to View, Add Event Theme. Fill in the appropriate information and click OK. Convert to a shapefile (this way you have a back-up copy), and project into State Plane 1927, Alaska zone 4 (this is the projection in which the Muni has its data). I suggest using the Projector! extension in the Files, Extensions list.

Inputting point data in ArcInfo

Use the GENERATE command to create a new coverage. Choose the POINTS keyword and follow the directions. Enter the ID number you want to use and then add the X,Y coordinates. Subsequent points get a new ID, then add the X,Y coordinates for each. BUILD the coverage using the POINT keyword. Reproject the data to State Plane, Alaska zone 4 (units is feet, and the datum is NAD27).

Inputting line data in ArcInfo

Use the GENERATE command to create a new coverage. Choose the LINES keyword and follow the directions. Enter the ID number you want to use and then add the X,Y coordinates in the order you collected them (hopefully this is the order in which they are in the real world). Add as many coordinates as you need for each feature. Subsequent lines simply get a new ID, then add the X,Y coordinates for it. BUILD the coverage using the LINE keyword. Reproject the data to State Plane, Alaska zone 4 (units is feet, and the datum is NAD27).

Inputting polygons data in ArcInfo

Use the GENERATE command to create a new coverage. Choose the POLYGONS keyword and follow the directions. Enter the ID number you want to use and then add the X,Y coordinates in the order you collected them (hopefully this is the order in which they are in the real world). If you capture a square, you will need to add 5 coordinates. For example, 1,1; 2,1; 2,2; 1,2; and 1,1. Be forewarned, if you don’t enter the original coordinate at the beginning and the end, you won’t get a square, you will get a triangle. Add as many X,Y coordinates as you need for each feature. Subsequent features simply get a new ID, then add the X,Y coordinates for it. BUILD the coverage using the POLY keyword. Reproject the data to State Plane, Alaska zone 4 (units is feet, and the datum is NAD27).

You can edit the feature attribute tables (FATs) at any time in the future. For example if you capture wells (using the first example above), you can always go back, add a field for depth, and attribute it.
You can use ArcInfo coverages in ArcView, but you cannot use ArcView shapefiles in ArcInfo. If you want to convert a shapefile to a coverage, use the SHAPEARC command and follow the directions in HELP.