GIS 317 Exam 1

Some nontopological editing operations can create features from existing features. Give two examples of such operations.

1) union features to combine features from different layers into one feature. this operation differs from the merge operation because it works with different layers rather than a single layer 2) merge features to group selected line or polygon features into one feature. If the merged features are not spatially adjacent, they form a multipart polygon, which is allowable for the shapefile and geodatabase

Which UTM zones is Kentucky in?

16N and 17N

The source map can greatly influence the quality of a digitized map. Explain how this is so.

A digitized map can only be as good or as accurate as the source map. If features are not accurately placed on the original, they will not be accurate on the digitized version. Paper maps often shrink or expand with change to the temperature and humidity. The quality of the line work is also important in the accuracy of the digital map; it should be thin, continuous, and uniform. Pencil maps should not be scanned but may be manual digitized due to smudging and erased lines showing up when scanned.

Explain the relationship between geodatabase, feature dataset, and feature class.

A feature class is a data set that stores features of the same geometry type. A feature dataset is a collection of feature classes within a geodatabase that share the same coordinate system and area extent. The geodatabase is an object bases vector data model. The heirarchical structure of the geodatabase allows for increased organization of projects and allows for seperate storage areas of data. If a project includes two study areas two feature datasets can be used to store feature classes for each study area.

Feature datasets are useful for data management. Describe an example (other than those in section 3.4.3) where it would be useful to organize data in feature datasets.

A feature dataset for watersheds might contain feature classes for streams, vegetation, soil types, and a DEM or TIN for elevation - these are all features that will be needed for hydrological analysis on the watershed.

Explain the importance of the map projection.

A map projection is the transfer of geographic coordinates from an ellipsoid to a plane. several things are affected based on map projection including distance, shape, direction, and size. it is important to consider the purpose of a map when selecting a map projection because while 1 or 2 of the previously mentioned factors can be preserved with certain selection the remaining will be distorted

What is pyramiding?

A technique that builds different pyramid levels for displaying large raster data sets at different resolutions. Resampling is used with pyramiding to build different pyramid levels. It is a multi-resolution technique that uses larger cell sizes with lower-resolution rasters (higher pyramid levels) to save memory & cause the raster to display faster.

What kinds of data must exist in a text file so that the text file can be converted to a shapefile?

All feature data is geospatial. Specifically, it must contain x,y coordinates in latitude & longitude or in a projected form.

What are the basic elements of the raster data model. Be descriptive vs. just listing.

Answer: 1)Cell Value: can be an integer or floating point. an integer has no decimal values and is usually representative of categorical data. Floating points can have decimal values and require larger amounts of storage. 2)Cell size: resolution. small cell has finer resolution 3) Raster Bands: single cell has only one value or multiband has multiple cell values at each location 4)Spatial Reference: align spatially with other data sets using x,y coordinates

arcs

Arcs are a series of line segments between nodes.

Explain the relationship between cell size, raster data resolution, and raster representation of spatial features

Cell size determines resolution, and the smaller the cells the higher the resolution. Large cells (low resolution) will not display fine features as well, because single cells will contain multiple values and the raster can only assign one value per cell.

Suppose a point location is recorded as (575729, 5228382) in data set 1 and (575729.44, 5228382.11) in data set 2. Which data set has a higher precision? In practical terms, what does the difference in data precision in this case mean?

Data set 2 has higher precision because it has more decimal places. It records location to the 100th of a unit (feet, miles, whatever), while data set 1 records location only to the nearest unit. In practical terms, data set 2 can record feature locations 100 times more precisely than data set 1.

What kinds of data are contained in the USGS DLG files?

Digital Line Graphs contain digital representations of point, line, and area features from USGS quadrangle maps at different scales. They include contour lines, hydrography, boundaries, transportation, and the U.S. Public Land Survey System and associated attribute data

What types of GPS data errors can be corrected by differential correction?

GPS data errors that can be corrected by differential correction are noise errors. Noise errors include: ephemeris (positional) error, clock errors (orbital errors between monitoring times), atmospheric delay errors, and multipath errors (signals bouncing off obstructions before reaching the receiver).

Define geospatial data.

Geospatial data includes both features (locational information) and attributes (characteristics of features). Geospatial data is data that is geographically referenced either through a geographic coordinate system using latitude and longitude or through a projected coordinate system using x-, y- values.

Describe an application from your discipline in which a GIS can provide useful tools for building a model.

I am currently researching and trying to develop an accurate methodology for calculating carbon sequestration in "urban forests" in which tree populations contain a mixture of both native and non native ornamental species. It could potentially be helpful to develop a map which pinpoints trees coordinates on a map with georeferenced data about each trees individual DBH, type, and age. I am equally interested in city planning and urban geography. It might be interesting to run a comparison on the presence of trees in an urban area vs the amount of pedestrian traffic.

Explain the difference between point mode and stream mode digitizing.

In point mode, the operator selectes points to digitize, whereas in stream mode lines are digitized at a preset time or interval. Point mode is better for straight-line segments, while stream mode is better for curves.

How many SPC zones does Kentucky have? What map projections are those SPC zones based on?

Kentucky has two State Plane zones, north and south. They both use Lambert Conformal Conic projections. There is also a new single zone for Kentucky.

Explain the difference between location errors and topological errors.

Location errors relate to the geometric inaccuracies of spatial features, while topological errors relate to logical inconsistencies between spatial features. Location errors are corrected by re-shaping or re-drawing features, while topological errors can be fixed with the tools included in a topology-based GIS package.

How is the scale factor related to the principal scale?

Map scale is not static; It is not the same everywhere on the map. It depends on the projection used and how the projection distorts the world. Scale is a ratio of a real world distance to a distance measured on the map. The principle scale, however, applies only to the standard line. On other parts of the map, the scale is called the local scale and is likely different from the principle scale. The scale factor is the ratio of the local scale to the principle scale.

What is map topology? (Don't answer with the definition out of the text. Explain how it pertains to GIS)

Map topology is the relationships between features in a vector map. If a map has topology, relationships between multiple features are saved as a single data set instead of multiple datasets. if two polygones shared a boundary instead of boundary being drawn twice it would only be drawn once. This allows for smaller data files and more accurate maps.

Describe two common datums used in the United States.

NAD27, a locally based datum based on the clarke 1866 ellipsoid, which was ground measured where measurements are affected by topography. In 1986 yhe national geodetic survey introduced NAD83 based on the GRS80 ellipsoid. in the case of the GRS80 ellipsoid the shape and size of the earth were determined through measurements made by Doppler sattelite observations.

Which of the three is recommended for categorical data. Why?

Nearest neighbor -- It preserves the original data values, so only the original categories appear as cell values in the transformed image. When data is categorical, operations such as averaging are inappropriate, because the values are really more like names than numbers.

Explain how differential correction works.

Noise errors in the satellite data for GPS cause incorrect positions for the data. To correct the error for the spatial reference, differential correction is used. This correction method uses points that have been accurately surveyed and have a known location. The difference from the predicted data (on the GPS) and the actual data becomes an error correction factor. The error correction factor is then applied to the data to shift all the data to the correct position.

Name the three types of simple features used in GIS and their geometric properties.

Point which has zero dimension Line which is one dimensional and has the property of length Polygon which is two-dimentional and has the properties of area (size) and perimeter, in addition to location.

How are control points selected for a map-to-map transformation?

Select well-distributed, easily recognizable points with known real-world coordinates. If real world coordinates are not known then points with known latitude and longitude coordinates can be used.

Name two examples each for integer rasters and floating point rasters

Soil type or habitat suitability (low, medium, high probability of finding a rare speices) would be integer rasters, and elevation and precipitation would be floating point rasters.

What kinds of data are contained in the TIGER/Line files?

TIGER/Line files are offered through the US Census Bureau and are extracts of geographic and cartographic information. Data that can be contained in TIGER/Line are legal and statistical area boundaries. This includes counties, census tracts, block groups, roads, railroads, streams, water bodies, power lines, and pipelines. Address ranges for different sides of the street are also available in TIGER/Line files.

5. According to the new National Standard for Spatial Data Accuracy, a geospatial data producer is encouraged to report a RMS statistic associated with a data set. In general terms, how does one interpret and use the RMS statistic?

The RMS is used when checking coordinates in a data set with the equivalent features in a data set of higher accuracy. When the RMS is multiplied by 1.7308, the resulting RMS statistic represents the standard error of the mean at a 95 percent confidence interval. The lower the RMS and RMS statistic, the better

What is SSURGO?

The Soil Survey Geographic database is maintained by the Natural Resources Conservation Service and provides field maps designed for use at the farm, township, and county levels.

Name two commonly used projected coordinate systems that are based on the transverse Mercator projection.

The State Plane Coordinate System uses the transverse Mercator projection in some zones. UTM Coordinate System is based on the transverse Mercator projection.

Edge matching requires a source layer and a target layer. Explain the difference between the two types of layers.

The distinction is that the target layer is assumed to be more accurate, and the features on the source layer are moved to match the locations of the equivalent features on the target layer along the boundary where the two tiles meet.

What is the difference between map-to-map and image-to-map transformation?

The former applies to a newly digitized vector map, which is originally measured in the units used by the digitizing table (e.g. inches). Map to map transformation converts it to a projected coordinate system. The latter applies to a raster image, which is originally in a grid measured by rows and columns. Image to map transformation converts it to a projected coordinate system and corrects geometric errors caused by motion of the satellite sensor.

The scanning method for digitizing involves both rasterization and vectorization. Why?

The map must first be scanned as a raster, which assigns a black/white or color value to each pixel. This raster is then vectorized through line thinning, line extraction, and topological reconstruction.

line simplification

The process of simplifying or generalizing a line by removing some of the line's points.

A sphere, a spheroid and a datum are all approximations of the shape and size of the earth. What are the key differences between them?

The sphere is the simplest model of the earth which approximates the shape and size of the earth. a better approximation of the shape of the Earth is the spheroid which takes into account the fact that the earth is wider along the equator than between the poles. A datum is a mathematical model of the earth which serves as the reference or base for calculating the geographic coordinates. a datum consists of an origin, an ellipsoid or spheroid, and the separation of the earth and the ellipsoid at the orgin.

Explain the difference between the standard line and the central line.

The standard line is the line where the projection surface touches or intersects the reference globe. There is no distortion along the standard line. The central line is the center of the map projection.

Although the U.S. National Map Accuracy Standard adopted in 1947 is still printed on USGS quadrangle maps, the standard is not really applicable to GIS data. Why?

The standards were designed for specific map scales (e.g. 1:100,000 has one standard, 1:24,000 has another). GIS is able to display and map data to any scale, so the threshold values for specific scales are not relevant.

Explain the advantages and disadvantages of the raster data model vs. the vector data model.

The vector model is better at displaying discrete features with well-defined boundaries, while the raster model is better for displaying continuous data that is highly variable over space. Rasters are easy for computers to manipulate as they are arrays of numbers and that's what computers work with, but they can take up a lot of storages space if the resolution is high. They also don't store as much attribute information as vector models, since the cell value typically represents one thing (elevation, cover type, etc.)

Explain the difference between the georelational data model and the object-based data model.

These terms refer to different data structures used to store data within a GIS. The georelational data model is a vector data model that uses a split system to store geometries and attributes and an object-based data model is a data model that uses objects to organize spatial data and stores geometries and attributes in a single system.

Explain the role of control points in an affine transformation.

They are used to determine the coefficients of the equations that are then used to convert the image to a projection or coordinate system.

Explain the difference between vector data and raster data.

Vector data uses points lines and polygons to represent spatial features or objects with a clear spatial location and boundary. The raster data model uses a grid and grid cells to represent continuous fields such as elevation and precipitation. Each cell has a value that captures the magnitude of the field at the cell location. The vector data model works well with discrete data while the raster model works well with continuous data.

What is vectorization?

Vectorization converts raster data to vector data through line thinning, line extraction, and topological reconstruction.

Explain the difference between the lossless and lossy compression methods.

a lossless compression preserves the cell or pixel values and allows the original raster or image to be precisely reconstructed. lossless compressions are ideal for raster data that is used for analysis or deriving new data. a lossy compression cannot reconstruct fully the original image but can achieve higher compression ratios. lossy is therefore better for raster data that is used for back ground images rather than analysis. an example of the lossy would be a jpeg where an image is broken down into blocks of 64(8x8) and process each block independently. colors in each block are shifted and simplified to reduce amount of coding

pseudo node

a node appearing along a continuous arc

coverage

a topological vector data format used in esri products

forms of topology by the coverage data model

connectivity, contiguity, area definition(containment)

What is the fundamental difference between a geographic and a projected coordinate system? (Note: do not simply list various categories of each found in box 2.5)

geographic systems are 3-D reference systems that use latitude and longitude. Projected coordinate systems are 2-D and use the map projection.

Which innovation made GIS much more user-friendly

geographical user interface

Table 3.1 shows "must not intersect" as a topology rule for line features. Provide an example from your discipline (stating which discipline you are in) that can benefit from enforcement of this topology rule

geography contour lines

Define geometries and attributes as the two components of GIS data.

geometries describe the location of spatial features (may be discrete or continuous). Attributes describe the characteristics of spatial features.

8. How are ground control points selected for an image-to-map transformation?

ground control points are points where both image coordinates and real world coordinates can be identiified. GCP's are selected directly from a satellite image therefore the selection is not as straight forward as selecting four tics for a digitized map. examples include road intersections, rock outcrops, small ponds, distinctive features along shorelines.

components of a gis

hardware, software, people, infrastructure

Explain the importance of topology in GIS

it ensures data quality and integrity. enables the detection of lines that do not meet. topology removes gaps or overlaps. if a gap exists on a supposedly continuous road, a shortest path analysis will take another route to avoid the gap. topology prevents this. second, topology can enhance GIS analysis. Provides further detail.

When is RMS error not a reliable indicator of the goodness of map-to-map transformation?

longitude and latitude readings printed on paper maps are sometimes incorrect. this would lead to to acceptable RMS errors but significant location errors for digitized map features.

Use an example from your discipline and explain the usefulness of integrating vector and raster data.

my current research involves trying to calculate wkus carbon sequestration by our urban trees. if i were able to calculate the rate of sequestration for various parts of campus i could represent that using a raster model and use the vector model to represent the various locations and density of trees on campus.

What is on-the-fly projection? When is it used? When should it not be relied on?

on the fly projection is used for displaying data sets that are based on different coordinate systems. on the fly projection does not actually change the coordinate system of the data set and can not replace projection and reprojection. it should not be relied on if the data set is to be used frequently in a different coordinate system or if the data set is to be used in spatial analysis.

What are the main data sources for digitizing? Which of these are primary data sources and which are secondary? What's the difference between a primary and a secondary data source?

primary being raw data and secondary being derived or compiled data. While the most common data source is paper maps, this is secondary data. The question uses the term primary in its technical sense, so the answer is GPS data and remote sensing imagery.

What do you do if the RMS error is too high?

remove the control points that contribute the most to the RMS error and replace them with new ones. continue the process until a satisfactory RMS error is attained.

Explain the role of the RMS error in an affine transformation. (That is, when is the RMS value acceptable? For a 1:24,000 topographic map? What assumption is made based on RMS error?)

the location of a control point on a digitized map or an image is an estimted location and can deviate from its actual location. a common measurement of the goodness of the control points is RMS error., which measures the deviation of between the actual and estimate locations of the control points. if the scale of the map is 1:24,000 a RMS of <6 is probably acceptable. The assumtion for the RMS error is that the level of accuracy can be assumed for the entire map which is not always the case.

line smoothing

the process of smoothing a line by adding new points, which are typically generated by a mathematical function such as a spline, to the line.

What are the main advantages of using shapefiles?

they can display more rapidly on the computer than topology based data. they are non proprietary and inter operable, meaning they can be used across different software packages

Describe the four types of map projections by the preserved property.

~Conformal - Preserves local angles and shapes. ~Equal Area (Equivalent) - Represents areas in correct relative size. ~Equidistant - Maintains consistency of scale along certain lines. ~Azimuthal (True Direction) - Retains certain accurate directions.

Why are there many different datums?

~Different datums are fitted to different portions of the Earth's surface, allowing accurate representation of feature location in those areas. Also, measurements of the Earth's shape have been refined over the decades, allowing newer, more accurate datums to be created. ~There are many different datums because many countries wish to obtain a better fit of the geoid local to their country and thus they create their own datum. Examples are the European Datum, the Tokyo Datum, and the North American Datum.

Name two examples of vector data analysis.

~Distance measurement which calculates distances between spatial features. ~Spatial statistics which detects spatial dependence and patterns of concentrations among features.

Describe two common types of field data that can be used in a GIS project?

~Land survey data constist of distances, directions, and elevations measured using tapes and other field instruments. It typically provides data for determining parcel boundaries. ~ Global positioning system receivers collect data using satellites to determine their position on the earth's surface. Positions can be used as point features or combined to form line or area features.

Describe the three types of map projections by the projection or developable surface.

~Projections can be visualized by imagining some type of planar surface is placed next to the globe (or slicing through it), shining a light from the center of the globe, and tracing the graticule and geographic features on the planar surface. An azimuthal projection keeps the planar surface flat, a cylindrical projection wraps the surface into a cylinder, and a conic projection wraps the surface into a cone. ~Case is defined by weather the developable surface is tangent to the globe or intersects the globe. The first is the simple case which results in one line of tangency. the second is the secant case which results in two lines of tangency. distortion is reduce near lines of tangency. the aspect describes placement of the relative surface relative to the globe. a plane can be tangent at any point on the globe. a polar aspect refers to tangency at the poles. equatorial n the aspect refers to tangency at the eequ

Suppose you are asked to convert a paper map to a digital data set. What methods can you use for the task? What are the advantages and disadvantages of each method?

~digitizing errors in orrigional map can be corrected during digitizing and people can easily recognize the map features. However it is slow and tedious and the quality of the results depends on the person who is digitizing ~scanning faster than digitizing with fewer errors from human fatique or inattention. however, features may not be correctly recognized during automated line tracing and the edditing process can take a long time.

Describe three common resampling methods for raster data.

~nearest neighbor: fills each pixel of the new image with the nearest pixel value from the original image. ~bilinear interpolation: uses the average of the 4 nearest pixels ~cubic convolution: uses the average of the 16 nearest pixels cubic convolution tends to produce a smoother generalized output than bilinear interpolation but requires a longer processing time.