1. Midterm II Multiple choice (46 points) Short answer (54 points)

2. GPS Remote sensing Spatial analysis

3. GPS 1.System consists of three segments 2. How is works (Triangulating, distance measure, timing, satellite position) 3. Error Sources (Satellite errors, atmospheric, Multi- path distortion, receivers error) 4. Error Correction (point averaging, Differential Correction

5. How It Works 1. Triangulating Start by determining distance between a GPS satellite and your position Adding more distance measurements to satellites narrows down your possible positions

6. Triangulating Three distances = two points Intersection of Four spheres = one point Note: 4th measurement not needed Used for timing purposes instead

7. 2. Distance measure Distance between satellites and receivers determined by timing how long it takes the signal to travel from satellite to receiver How? Radio signals travel at speed of light: 186,000 miles/second Satellites and receivers generate exactly the same signal at exactly the same time Signal travel time = delay of satellite signal relative to the receiver signal Distance from satellite to receiver = signal travel time * 186,000 miles/second 1  sec Receiver signal Satellite signal

8. How do we know that satellites and receivers generate the same signal at the same time? satellites have atomic clocks, so we know they are accurate Receivers don't -- so can we ensure they are exactly accurate? No! But if the receiver's timing is off, the location in 3-D space will be off slightly... So: Use 4th satellite to resolve any signal timing error instead determine a correction factor using 4th satellite 3. Getting perfect timing

9. 4. Satellite position In order to make use of the distance measurements from the satellites, we must know their exact locations such that we can match our signals with the right satellite. satellites are placed into high orbits -- makes their orbits very predictable receivers have almanacs that tell them where satellites should be minor variations in orbit are monitored -- correction factors transmitted along with the signals

10. Error Sources Satellite errors satellite position error atomic clock, though very accurate, not perfect. Atmosphere Electro-magnetic waves travels at light speed only in vacuum. The ionosphere and atmospheric molecules change the signal speed. Multi-path distortion signal may "bounce" off structures nearby before reaching receiver – the reflected signal arrives a little later.

11. Error Sources (cont’d.) Receiver error: Due to internal noise. Selective Availability intentional error introduced by the military for national security reasons  Pres. Clinton cancelled May 2, 2000.

12. Error Breakdown (typical case): satellite clock: satellite orbit: ionosphere/troposphere: multipath distortion: receiver errors: 1.5 meters 2.5 meters 5.5 meters 0.6 meters 0.3 meters

13. GPS - Error Correction 2 Methods: Point Averaging Differential Correction

14. Remote sensing 1.What is remote sensing? Why remote sensing? 2.Active system and passive system 3. Electromagnetic radiation energy 4. Characteristics of remote sensing data 5. Two types of sensors (across-track scanning and along-track scanning) 6. NDVI 7. Image classification 8. Remote sensing applications

15. What is Remote Sensing? Remote  far away Sensing things from a distance Remote sensing is the science and art of obtaining information about a target through the analysis of data acquired by a device that is not in contact with the target under investigation. What we see & why Eyes: Sunlight is reflected onto our nerve cells in the retina. What we see: Visible spectrum (blue, green, red wavelengths) Remote sensing equipment allows us to sense electromagnetic radiation beyond the visible spectrum

16. Remote Sensing Sensor Systems 1.Active systems: The active sensors generate and transmit a signal toward the garget and receive and record the returned signal. Examples: Radar, LIDAR (Light Detection and Ranging) 2. Passive systems: The passive sensors do not generate or transmit a signal, however, they detect and record the natural electromagnetic energy reflected and/or emitted from a target. Examples: Cameras, radiometers

17. Solar Radiation Electromagnetic radiation energy: Wave-particle duality. Particle=photon Wavelength Light speed: c=f c = speed of light (186,000 miles/second) f = light frequency: number of waves passing a reference per unit time (e.g., second). The amount of energy carried by a photon:  = hf h=Planck’s constant (6.626  10 -34 Js)

18. Solar Electromagnetic Radiation Atmospheric windows

19. Satellite Remote Sensing Resolutions Spatial: Area visible to the sensor Spectral: Ability of a sensor to define fine wavelength intervals Radiometric: A bility to discriminate very slight differences in energy Temporal: Amount of time before site revisited

20. Vegetation Information Normalized Difference Vegetation Index NDVI: [-1.0, 1.0] Often, the more the leaves of vegetation present, the bigger the Contrast in reflectance in the red and near-infrared spectra.

21. Multiple choice 1.How many satellites are required for determining the position of a GPS receiver? A.3 B.4 C. 5 D. 6

22. 2.Spectral resolution of a remotely sensed image refers to: (A) The number of pixels in an image. (B) The number of sensors that a remotely sensing device has. (C) The number of images that covers the same areas. (D) The number of bands and bandwidth in which the remotely sensed data are collected.

23. An 120km by 120 km image frame one thermal band with 120m spatial resolution Six other bands with 30m spatial resolution 8 bits per pixel How many bytes for the image?

24. What is NDVI? Why water has low NDVI value (i.e., negative value)?

25. Analysis is considered spatial if the results depend on the locations of the objects being analyzed. Thus if you move the objects, the results of spatial analyses will change. To conduct a spatial analysis generally requires both attributes and locations of objects. Spatial Analysis

26. Steps in Spatial Analysis 1.Frame the question we wish to ask. 2.Find appropriate data to answer the question. 3.Choose analytical method appropriate to answer question. 4.Process the data using the chosen method. 5.Review and question the results (often refining the approach by collecting more data and improving the analysis)

27. Spatial Relationships are at the Core of Spatial Analysis Most spatial analyses are based on topological questions: How near is Feature A to Feature B What features contain other features? What features are adjacent to other features? What features are connected to other features? From these topological building blocks, we can develop all sorts of spatial analysis approaches to answer many complex questions

28. Types of Spatial Analysis We will consider three categories of spatial analyses: 1. Queries 2. Measurements 3. Transformations

29. 1. Queries Queries Attribute based Example: show me all pixels in a raster image with BV > 80. Location based List all the block groups that fall within Orange County A GIS can respond to queries by selecting the appropriate data in A map view A table Both

32. Age <=20

33. ‘Male’ And ‘Age < 20’

34. From same source village

35. 2. Measurements Measure: Distance between two points Distances can be summed –Example: a truck makes multiple stops on a route. What is the total distance traveled on the route? Area of a polygon –Example: What is the area of a preserved forest tract?

36. Measurement of Length Types of length measurements Euclidean distance: straight-line distance between two points on a flat plane (as the crow flies) Manhattan Distance limits movement to orthogonal directions Great Circle distance: the shortest distance between two points on the globe Network Distance: Along roads Along pipe network Along electric grid Along phone grid By river channels

37. Distances can be calculated between points, along lines, or in a variety of fashions with areas Euclidean Distance – calculated in a Cartesian frame of reference: Euclidean Distance (x 1 – x 2 ) 2 + (y 1 – y 2 ) 2 C= C P 1 (x 1,y 1 ) P 2 (x 2,y 2 ) On what scales is this valid? Can we use this with latitude and longitude?

38. Manhattan Distance is useful in some urban environments with orthogonal road networks. Movement is limited to city streets: Manhattan Distance P 1 (x 1,y 1 ) P 2 (x 2,y 2 ) d m = | x 1 – x 2 | + | y 1 – y 2 | a reminder – the | symbols denote absolute value

39. Great Circle Distance The Great Circle distance is the shortest distance between two points on the globe. The two points must be specified using latitude & longitude positions.

40. Starting from: Carrboro, NC 27510 Save Address Arriving at: Washington, DC Save AddressSave Address istance:272.5 miles Approximate Travel Time:5 hours 23 mins Network Distance Yahoo maps:

41. Issues with Length Measurement The length of a true curve is longer than the length of its polyline or polygon representation:

42. Issues with Length Measurement Length measurements in GIS are usually calculated in 2 dimensions. But changes in elevation increase distances. X Z

43. 3. Transformations Spatial transformations includes many analytical approaches, applicable to: Vector data Raster data Both Transformations can create new: Attributes Data objects

44. Buffering (Proximity Analysis) Buffering operations create new objects consisting of areas within a user-defined distance of existing objects. Examples of uses: to determine areas impacted by a proposed highway to determine the service area of a proposed hospital Buffering can be performed in both the vector and raster spatial data models

45. Buffering: The delineation of a zone around the feature of interest within a given distance. For a point feature, it is simply a circle with its radius equal to the buffer distance: Buffering (Proximity Analysis)

46. The buffer zone constructed around each feature can be based on a variable distance according to some feature attribute(s) Suppose we have a point pollution source, such as a power plant. We want to zone residential areas some distance away from each plant, based on the amount of pollution that power plant produces For smaller power plants, the distance might be shorter. For larger power plants that generate a lot of pollutant, we choose longer distances Variable Distance Buffering

47. Buffering higher order objects involves moving a circle of specified radius along the line (or the lines forming polygon) Buffering Points, Lines, and Polygons

48. Buffer lines Buffer polygons Line and Polygon Buffer Examples

49. Raster Buffering Buffering operations also can be performed using the raster data model In the raster model, we can perform a simple distance buffer, or in this case, a distance buffered according to values in a friction layer (e.g. travel time for a bear through different landcover): lake Areas reachable in 5 minutes Areas reachable in 10 minutes Other areas

50. Feature in Feature Transformations These transformations determine whether a feature lies inside or outside another feature The most basic of these transformations is point in polygon analysis, which can be applied in various situations:

51. We can use point in polygon results to calculate frequencies or densities of points per area For example, given a point layer of bird’s nests and polygon layer of habitats, we can calculate densities: Habitat Area(km 2 ) Frequency Density. A 150 4 0.027 nests/km 2 B 320 6 0.019 nests/km 2 C 350 3 0.009 nests/km 2 D 180 3 0.017 nests/km 2 Bird’s Nests AB DC Habitat Types AB DC Analysis Results Point Frequency/Density Analysis

52. Overlay line layer (A) with polygon layer (B) –In which B polygons are A lines located? » Assign polygon attributes from B to lines in A A B Example: Assign land use attributes (polygons) to streams (lines): Line in Polygon Analysis David Tenenbaum – GEOG 070 – UNC-CH Spring 2005