GIS EXAM #2 QUERIES. Attribute queries only looks at the records in the attribute tables to some kind of

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1 GIS EXAM #2 QUERIES - Queries extracts particular records from a table or feature class for use; - Queries are an essential aspect of GIS analysis, and allows us to interrogate a dataset and look for patterns; - Important for extracting features to create a new layer or for ongoing analysis; Two main types of queries Attribute queries only looks at the records in the attribute tables to some kind of condition. These kind of queries can operate independent of spatial data. Attribute selection is based on a logical expression in a language called Structured Query Language or SQL a language common to many database programs. Boolean operators And meets one criteria AND another; Or meets one criteria OR another; Not Meets the condition of one property and ensures exclusion of any that meet the Not category; Xor the exclusive or operator meets both conditions, except where there is overlap (rarely used). Partial operators Like a wildcard type operator allows select parts or some of a criteria (a fuzzy search that s not always easy to apply). Spatial queries only looks at the location information, instead of the attribute tables. These queries involve distances, locations within areas (e.g., volcanoes within 20miles of a highway). Containment features are located within or partially within a polygon area...others are available; Intersection identifies features that touch cross or overlaps any part of a feature; Proximity identifies features in layer A that are in a given proximity to layer B.

2 After a query is carried out, any further operations are carried out on only the subset, usually highlighted in a light blue. A table or layer may be in one of three selection states; 1.)No selection (default state) 2.)No features selected (zero records selected) 3.)Some features selected (some returned number of selected features); SPATIAL JOINS - Spatial joins involves the joining of points, lines and polygon attribute data using locations. - The process is different to attribute joins, which use tabular data and common fields (e.g., the FIPS field) to perform a join; - Spatial joins require x,y locations to perform the join, instead of a common attribute field; - Usually UTM measures will be the most common coordinates for these applications. Types of Spatial Joins; 1. Inside simple; (e.g., which points fall in which polygon); 2. Inside summarized; (e.g., how many points fall within each polygon); 3. Simple distance based spatial joins; which point is nearest each other point (for example, fire-hydrants next to houses); 4. Summarized distance based spatial; how many specified point locations closest to other specified point locations; Possible join types; Points to points simple distance, summarized distance; Lines to points simple distance, summarized inside; Polygons to points simple inside, simple distance; Points to lines simple distance, summarized distance; Lines to lines summarized inside simple inside; Polygons to lines summarized inside, simple distance; Points to polygons summarized inside, simple distance; Lines to polygons summarized inside, simple distance; Polygons to polygons summarized inside, simple inside. Simple inside join: each point will acquire the attributes of the associated polygons.

3 Reversal of this join will produce a summarized inside join or how many schools per polygon, other combined and summarized attributes may also be included in these kinds of spatial joins. A Simple distance join will involve some kinds of distance based measure to link the two feature datasets. A summarized distance join involves counting the number of attractions closest to each other location. For example, find all towns closer to one hotel than another hotel. GeoPROCESSING Map overlay procedures; -Extraction approaches those that extract one layer based on the properties of another. -Combination approaches those that combine two or more layers together. Clip: a cookie cutter approach where one layer is subsetted based on the extent of another. The procedure is often used to clip data to some study area boundary. Erase: A procedure that is opposite to a clip procedure; keeping all features outside of an erase area. Intersect: A procedure that combines layers and retains information common to the intersected areas. Union: Retains information from both layers entirely.

4 Dissolve-Used when you want to eliminate complexity in a spatial database. The process groups information together when they are neighbors and share the same attribute. Buffer- A process that defines/identifies areas within a specified distance of a coverage (e.g., within 300m of a stream),process is applicable to points, line and polygons. Append: Stitches together layers of the same type and coordinate system resulting in a single mega layer. Merge: stitches data together as a way reducing file numbers and creating a large (mega-area) dataset based on two or more files. Offers greater flexibility than append as the attribute tables are not required to be the same. Ways to perform geoprocessing Arctoolbox: Simple way to integrate the tools and explore procedures. Command line: more advanced way and may lead to script languages, for those computer programmers in the course. Model builder: Graphical way of stitching together commands. Scripts: Customized text based sequence of commands creating a small program. Similar to the command line or a command-line version of the Model builder. Geoprocessing considerations Environments, Coordinate systems, Areas & lengths, GPS The satellite constellation is comprised of 27 Earth-orbiting satellites; however, only 24 are in continuous operation; The remaining three are available on standby if general maintenance and failure requires a satellite backup. The network of 24 satellites Navstar was placed into orbit by the U.S. Department of Defense; The system was setup in 1978 and used for military applications (e.g., cruise missiles, pinpoint enemy locations and eliminate civilian casualties).

5 How does it work? Twice a day, the GPS satellites circle the Earth in an extremely precise and predefined orbit at an altitude of 11,000 miles; Each satellite continuously transmits a time signal to the Earth; GPS receivers (e.g., hand-held systems) use these time signals along with triangulation mathematics, to calculate the users exact location. To perform this operation, the GPS receiver must be locked on to the signal of at least three satellites to calculate a 2 dimensional (2D) position (latitude and longitude); Four or more satellites are required to determine the user's 3D position (latitude, longitude and altitude or elevation). Triangulation Process mentioned earlier specifically the 2D and 3D process. The technical term for this process is Trilateration. Once a position is been determined, the GPS unit can calculate other information, such as bearing, speed, trip distance, distance to destination, and local features such as sunrise and sunset time. How accurate are GPS s? Fine Print: Atmospheric factors (e.g., dust and moisture) and other sources of error can affect the accuracy of GPS receivers. Garmin.. and most hand-held off the shelf GPS receivers are accurate to within 15 meters on average. Accuracy is generally to within 10m (30 feet) of the actual for x, y. Double the x, y error for the vertical (65 feet). Some of the newer GPS receivers are equipped with a system known as WAAS (Wide Area Augmentation System). This can improve the positional accuracy to less than three meters on average; WAAS is essentially an established form of differential GPS. What is Differential GPS? Differential GPS (DGPS) corrects GPS signals to within an average of three to five meters. Again this is a conservative measure and it can improve with careful calibration and setup; DGPS involves a network of precisely located towers or base stations. These towers receive GPS signals and transmit a corrected signal using a beacon transmitter.

6 PDOP (Percent Dilution of Precision) Gives you an indication of the amount of error in your position; When the satellites are not distributed widely across the skyline (i.e., they are clustered in a small area) or if they are too low in the horizon, the mathematical calculations produce inaccurate measurements. PDOP less than 4 gives the best accuracy (under 1 meter) error; Between 4 and 8 gives acceptable accuracy; Greater than 8 gives poor accuracy; Real accuracy = Receiver accuracy x PDOP; Sometimes PDOP is known as GDOP (Geometric Dilution of Precision). Selective Availability (SA) When the GPS was initially released, civilians could only receive satellite signals which could generate locations to within 100 meters (332 feet). This was strategic as the system was established for military purposes; May 2, 2000 Selective Availability was removed but could be reimplementation given future predicaments; SA On SA Off Other sources of error So without SA, what causes the measurement errors? Atmosphere (dust, weather, electronic activity in the upper atmosphere); The lower the satellite in the horizon the longer the signal has to travel through the atmosphere; Multi-path signals those bounced off walls, water, trees etc. which interfere with the time signal; Receiver errors primarily the receiver's clock.

7 GPS to GIS Various software packages (primarily supplied by the manufacturer of the GPS receiver) are used to transfer the logged data into a format suitable for the GIS software (e.g., Shape files). ArcPad A software tool for fieldwork and GPS data collection, storage and GIS data editing program designed for use in a Windows Pocket PC or Windows Mobile device; User interface looks very similar to that of ArcMap, except fits on a small screen and has fewer available functions. Handling GPS data (Starting point) Two commonly used reference systems (coordinates) 1. Latitude and Longitude Ensure you record and what datum are used as part of your meta data! Record what spheroid are used; Use the Geographic Coordinate System also known as lat and long; 2. Projected Cartesian x, y coordinates (e.g., UTM or Michigan GeoRef) Record the projection parameters such as the units (Meters, feet) and zone; Record which datum were used (e.g., Nad27, Nad83); Record the spheroid information; Importing an Excel file Go to ArcMap s menu Tools xy Data and add the excel file; It will only work if the number are in decimal degrees and not the geographic latitude and longitude coordinates; Import the coordinates and change the projection at the same time (Projected Coordinates; North America; North American Datum 1927); View the resulting points and export them to.shp files or a GeoDatabase on your thinkpad PC.

8 RASTER ANALYSIS Raster vs. Vector Raster data model provides a suitable surface analysis toolpak, yet both of the data models have advantages and disadvantages; GIS operators should have a good grasp of both types of data models; as to date much of our work to date has been vector based. The Spatial Analyst toolpak is an expensive add-on package for ArcGIS specifically used for handling raster data. Many raster formats are available, such as TIF, JPEG, GIF; ESRI uses two main models, the ERDAS Imagine (.img) format; ESRI GRID format; ESRI s Spatial Analysis uses the ESRI Grid Format which offers pixel values in: Integer based-grids (whole numbers in binary format); Floating point grids for decimal place data (4x larger file size); Signed values include positive & negative signatures (e.g., bathymetric data); Multiplication of data of different types introduces problems. Representing No Data Sometimes No Data are represented as 0 or or some other specified numbers; Otherwise these pixels will be coded specifically as No Data so the numerical pixel values do not interfere with raster-based calculations. GeoReferencing Ground Control Points -Actual locations, found on an image and used to link the pixel values to their real world locations. Raster analysis Map Algebra Numerical pixel values stored as layers can be combined to other layers with simple mathematical operations (e.g., addition, subtraction, multiplication). Pixels in the input layers will combine to create a new raster output layer. Pixels may involve other changes (e.g., imperial to metric units)

9 Boolean overlay Raster overlay functions which deliver a result based on the evaluation of input conditions; (e.g., avalanche_risk = elevations > 1000 and slope > 15.5). Distance function Calculations of straight-line (Euclidean) distance from features (e.g., distance to river) where each pixel will represent a distance to a particular location. Density Calculations that indicate the density of entities within a specific area. e.g., precipitation, pollution, population. Interpolation (raster) Using point data collected across a study area to evaluate numerical values over a surface. Examples include temperature values, elevation values, pollution values and so on. Interpolation methods are available 1. Inverse Distance Weighted (IDW) a local interpolation method that enforces a condition that unknown point values are influence more by nearby points than from those farther away; 2. Kriging a stochastic (random) interpolation method that assumes the spatial variation of an attribute includes a spatially coherent component. Surface analysis Applied to elevation data, this process will create surfaces such as slope, aspect and hillshading from a Digital Elevation Model (DEM). Cut & Fill to calculate the difference between two input surfaces may also be considered a type of surface analysis. Neighborhood statistics Operations that function in catchments specified areas. Can also focus on neighboring cell values from a target cell Moving window neighborhood operations, 3x3, 9x9 etc. will operate on target cells in the center of the window.

10 Reclassify Will form classifications of pixels or re-classifications of pixels; Classification of slope values into high, medium and low values; Conversion Converts between vector and raster data and vice versa. The field choice will be required for creating a specified type of raster data. Masks (Grid/Analysis) A system that is used to screen out areas of unwanted data; Could be seen as analogous to a clip coverage in a vector data environment; The mask will involve areas of NoData cells; Data within these areas are excluded from the analysis. Spatial analyst As you can see there are many components and intricacies associated with raster data; to cope with these intricacies, a separate set of tools, purchased separately and designed specifically to modify raster data. Many of the Spatial Analyst tools are available from the tool-bar, but many area also available within the ArcToolBox. Taking some time to look through the tools in Arctoolbox will give you a good practice, use the index function.