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GIS 121 - LEAF International Map

Here is our first map of the semester. It shows the program locations of LEAF International. The map was compiled using Batchgeocode.com to create tab delimited text file of the program cities. The tab delimited text file was added into the Map Documented and a shapefile was created from the XY coordinates.

Final GPS Exam Study Guide

Identify the 3 segments of GPS

• Control – ‘brain’ of the system
• Controller monitors system and sends adjustments if necessary.
• The DoD operates this segment from Falcon Air Force Base in Colorado Springs, Co.
• Also, 4 monitoring and upload stations throughout the world.

• Space
• NAVigation Satellite Timing And Ranging
• Fully operational system consists of at least 24 satellites
• Constantly being ‘commissioned’ and ‘decommissioned’
• 20,200km above earth, one revolution every 12 hours

• User
• GPS receivers on the ground
• Civilian users outnumber military. Many applications.
• Emergency Services, Public Safety, Recreation, Asset Management, Routing, Agriculture, Vehicle Tracking

List the acceptable number of satellites (SV's - space vehicles) for an accurate 3D position.

• GPS position is calculated by Trilateration, measuring of the distance from at least three satellites to establish a position on earth.
• Four for an accurate 3D postion.
  
• Trigonometry requires three perfect measurement to define a point in three-dimensional space. However, the accuracy of a measurement based on three satellites may be diminished due to the non-synchronization of clocks in the GPS satellites and receivers.

List and explain sources of error in the GPS system

• Selective Availability – turned of in May of 2000
• Obstructions – buildings, vegetation
• Multipath – GPS signals reflecting off objects
• Atmospheric delay
• Worse during the heat of the day
• Worse when satellites low on the horizon
• GPS works in all weather conditions

Explain why satellites are provide a good system for locating positions on the earth

• Accurate position can be determined without having a direct line of sight between the known and unknown location.
• Only need a clear view of the sky
• Can determine accurate position almost anywhere at anytime on the earth.

Define trilateration

• GPS position is calculated by Trilateration, measuring of the distance from at least three satellites to establish a position on earth.
• Four for an accurate 3D postion.
  
• Trigonometry requires three perfect measurement to define a point in three-dimensional space. However, the accuracy of a measurement based on three satellites may be diminished due to the non-synchronization of clocks in the GPS satellites and receivers.

Discuss how the distance from the satellite to the GPS receiver is determined.

• Two factors involved in measuring the distance from a satellite.
• The speed of the radio signal (the speed of light = 300,000 km/sec)
• The time it takes the signal to reach the earth
• Each GPS satellite transmits data that indicates its location and the current time.
• All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant.
• The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver.

Describe the almanac and what it is used for

• Set of parameters used to calculate the general location of each satellite.
• The almanac specifies where each GPS satellite will be at any given moment in the near future.
• Almanacs are used for planning the best time to collect GPS data and for quick acquisition of satellite positions by the receiver.

Define ephemeris information and what it is used for

• The orbits of all GPS satellites are measured constantly by the DoD.
• They determine satellite ranging and calculate the exact location of each satellite.
• The adjusted measurements are transmitted from the DoD to the satellites
• These minor corrections are then transmitted by the satellites as ephemeris information.
• The ephemeris information is a data file that contains orbit information for one particular satellite.
• Information used by the GPS receivers along with their internal almanac to establish precisely the position of a satellite.

Define autonomous GPS

• GPS without some form of data processing to insure accuracy (not corrected in the field). It is accurate to +/- 10 M.

Describe differential GPS (SBAS, WAAS, Post-processing)

• The precise measurement of the relative positions of two receivers tracking the same GPS signals.
• One receiver places at a ‘base’ station over a know coordinate
• Other receivers (rovers) use the corrected GPS information to eliminate error in their measurements
• The ‘base’ station receives the same GPS signals as the roving receiver but instead of working like a normal GPS receiver it attacks the equations backwards.
• Instead of using timing signals to calculate its position, it uses its known position to calculate timing.
• It figures out what the travel time of the GPS signals should be, and compares it with what they actually are. The difference is an "error correction" factor.
• The base receiver then transmits this error information to the roving receiver so it can use it to correct its measurements.
• Wide Area Augmentation System
• FAA’s strategic objective of a seamless satellite navigation system for civil aviation.
• WAAS is based on a network of approximately 25 ground reference stations that covers a very large service area.
• Signals from GPS satellites are received by wide area ground reference stations (WRSs).
• Each of these precisely surveyed reference stations receive GPS signals and determine if any errors exist.

Define GPX

• GPX, or GPS eXchange Format is an XML schema designed for transferring GPS data between software applications. It can be used to describe waypoints, tracks, and routes. The format is open and can be used without the need to pay licence fees.

What is a waypoint?

• Waypoints are sets of coordinates that identify a point in physical space. For the purposes of terrestrial navigation, these coordinates usually include longitude and latitude, and sometimes altitude (particularly for air navigation). Waypoints have only become widespread for navigational use by the layman since the development of advanced navigational systems, such as the Global Positioning System (GPS) and certain other types of radio navigation. Waypoints located on the surface of the Earth are usually defined in two dimensions (e.g., longitude and latitude); those used in the Earth's atmosphere or in outer space are defined in at least three dimensions (four if time is one of the coordinates, as it might be for some waypoints outside the Earth).

Describe the critical software settings in Trimble TerraSync

• PDOP = Position Dillution of Precision = 6 or higher (Measure of the geometry of the satellites in the sky)
• Elevation Mask = 15 degrees or more (Satellites must be 15 degrees or more above the horizon or they are not used)
• SNR = Signal to Noise Ration = Less than 39 (measure of the strength of the signal)

What is an .ssf file?

• The native format for data files stored on Trimble GPS units. It stores attribute information beyond what a recreational GPS could store.

Describe what Trimble Pathfinder Office is used for.

• Pathfinder Office is used to construct data dictionaries as well as to post-process data collected on Trimble units in the field. Pathfinder Office also aids in transferring files to and from the Trimble unit.

Describe several differences in a professional grade GPS like the Trimble GeoXT and a recreation grade GPS like the Garmin

• Professional grade GPS devices allow the user to filter for accuracy.
• Professional grade GPS devices allow data to be post processed.
• Professional grade GPS devices have a higher accuracy rate.
• Professional grade GPS devices allow for attributes to be stored during feature collection.
• Professional grade GPS devices allow for data to be collected through the use of offsets.
• Professional grade GPS devices allow for data to be collected through a greater variety of means.
• Professional grade GPS devices can be loaded with different operating environments to meet the needs of the user.

Exam 2 Study Guide

1. Define an attribute domain.
   
• In a geodatabase, a mechanism for enforcing data integrity. Attribute domains define what values are allowed in a field in a feature class or nonspatial attribute table. If the features or nonspatial objects have been grouped into subtypes, different attribute domains can be assigned to each of the subtypes.
  
  
• What are the 2 types?
• Coded-value and range
  
  
• How are range domains validated?
• Range domains are validated in an editing session through the Editor toolbar.
  
  
• What are split and merge policies?
• Split and merge policies define how the values of a field will react once a feature is split or merged (i.e. are the values added, is the highest selected, does the field defer back to the default value). Split policies include default value, duplicate, and geometry ratio. Merge policies include default value, sum value, and weighted average.
• 
  
  
2. Define a subtype.
• What data type must the field be to apply a subtype to it?
• In geodatabases, a subset of features in a feature class or objects in a table that share the same attributes. For example, the streets in a streets feature class could be categorized into three subtypes: local streets, collector streets, and arterial streets. Creating subtypes can be more efficient than creating many feature classes or tables in a geodatabase. For example, a geodatabase with a dozen feature classes that have subtypes will perform better than a geodatabase with a hundred feature classes. Subtypes also make editing data faster and more accurate because default attribute values and domains can be set up. For example, a local street subtype could be created and defined so that whenever this type of street is added to the feature class, its speed limit attribute is automatically set to 35 miles per hour.
  
  
• Objects in an object class and feature in a feature class may be further subdivided into subtypes.
  
  A subtype is a special attribute that lets you assign distinct simple behavior for different classifications of your objects or features. All subtypes of a class share the same set of attributes.
  
  The motivation for defining subtypes of an object class is to introduce a lightweight subdivision of an object class that adds these capabilities:
• You can name subtypes to describe each member of a classification of your objects.
• You can define distinct attribute domains for each field in a subtype.
• You can define distinct default values for each field in a subtype.
• You can prescribe the types of relationships that are possible between the objects in a subtype and objects in another subtype in the same or different object class.
• If you write some software code, you can also add custom rules for subtypes of object and feature classes.

An object class does not have to contain subtypes. If none are defined, you can still set attribute domains, default values, and rules - but on the object or feature class as a whole instead of a subtype.

• Subtypes are only permitted on Long Integer fields
  
  
• Why do we create subtypes?
• What are the advantages?
• Subtypes increase data integrity and can speed data entry.
  
• p. 88 in the text
1. Describe the spatial adjustment process in ArcMap for transforming data from one coordinate system (or non-real world coordinates) to another coordinate system.
• Georeferencing is used for CAD files and for images. When these are in non-real world they must have real world coordinates to display correctly. A world file is created with the same name as the image or CAD. When these are displayed in ArcMap, the world file is used to provide the coordinates. Georeferencing builds the world file.
  Spatial adjustment is a tool used in an editing session for vector feature classes to actually update the data with coordinates. These are three processes that can be used - transformation, rubbersheeting and edgematching. In class we used transformation. This used control points - coordinates that had been collected with a GPS unit. Because this is done in an edit session one can use tools such as snapping to improve the process.
  
  
2. Describe how CAD data is represented in ArcMap. Also see: Contents of CAD drawing.
   
• Define CAD feature datasets
• CAD feature datasets are an ArcGIS feature data object representation of a CAD drawing. They contain a collection of CAD feature classes that allow you to interact with CAD data in ArcGIS similar to how you would with geodatabases and shapefiles. This includes performing operations such as attribute query, symbol modifications, and conversion. However, CAD feature datasets are read-only, so their feature geometry and attributes cannot be edited.
  
  
• Define CAD feature classes
• CAD feature classes are homogeneous collections of CAD features of the same geometry type that possess a common set of attributes. Typical feature classes are geographical representations of features as points, lines, polygons, and multipatches; they can also be map text (annotation). All CAD feature class types will always exist in a CAD feature dataset, even if a feature class does not contain any features. You cannot add CAD feature classes to a CAD feature dataset.
  
  
• Define CAD drawing datasets
• CAD drawing datasets are a unique representation of a CAD drawing. They contain all the contents of the CAD drawing. This means that all geometries are organized into a single layer. This design allows you to work with a CAD drawing file as one entry in the catalog or map, similar to how you work with a drawing file in a CAD program.
  
  CAD drawing datasets reside within a CAD feature dataset and are denoted by a white compass icon. They are rendered using the original colors of the CAD entities. They have limited functionality when you compare them to CAD feature classes. You can turn drawing layers on and off, perform coordinate transformations, and modify the display effects (brightness, contrast, and transparency). However, you cannot change their symbols, perform spatial or attribute queries, or use them as a snapping layer.
  
  
• Understand the differences - which do we use to import to a GIS data format? Which one can we change the symbology of?
  
  
3. Describe what makes features smart. (chapter 5 in the text)
   
   
4. How do we progressively add intelligence to features in the Geodatabase?
   
   
5. What is the importance of the Spatial Reference in ArcGIS? XY domain in the Geodatabase? How is this different than with a Shapefile?
• XY DOMAIN - In Geodatabases, you have to specify the an XY domain for your feature classes. The XY Domain is the 'bounding coordinates' that your features can fall within. If you try to put a point, line, or polygon on the map that is outside these bounding coordinates, it will not let you add the geometry. You can not alter the XY domain after you create it so you must plan for as big as your data may grow geographically.
  
  Shapefiles do not require specifying an XY domain. You can create a shapefile and add a point in Asheville and one in Paris and one in India. The shapefile does not have a bounding box of coordinates the data must fall into. You can put points literally anywhere in the world.
  
  The underlying reason the geodatabase using the XY Domain, and we did not discuss this, has to do with how it stores data and allows the geodatabase to store more precise data because of the XY domain.
  
  
6. What do you know about Raster Catalog's
• A collection of raster datasets defined in a table of any format, in which the records define the individual raster datasets that are included in the catalog. Raster catalogs can be used to display adjacent or overlapping raster datasets without having to mosaic them together into one large file. In ArcView 3.x, raster catalogs were called image catalogs.
  
  Raster catalogs can be constructed from varying imagery types and resolutions. The imagery is referenced rather than mosaicked.
  

Digitizing

At the moment, work on the database is revolving primarily around digitizing the roads and sidewalks on campus. We declared domains and subtypes for the sidewalks in order to get practice with streamlining data creation.

Update on the Campus Geodatabase

Here's the latest update on the campus geodatabase.
We have been converting our CAD data into polyline data within our geodatabase and then converting the associated buildings into polygons. As there is a significant difference between our CAD data and the orthophotos, a lot of data will need to be digitized manually.

Geodatabase Structure

I just wanted to throw up a quick post as to capture the evolution of our class geodatabase. We began initial work on 2-October and the graphic below captures the organization thus far. At this point we are building our geodatabase without a predefined structure, and I will be doing my best to capture the relationships and dependencies that we find to be necessary as we move along in the process.