Lab 5: Digital Data

I. Questions based on lecture and book

1. Advantages of using digital spatial data include the following: it’s the easierst, quickest, and least expensive source for much spatial data. The disadvantage of digital spatial data is that because it is so readily available, you must check the source and metadata to make sure the data is reliable.

2. You must make sure that the coordinate projection system is the one that you need to use. You should also make sure that the data was collected and assembled by a reliable source. You should make sure that you’re using the most up-to-date version of that data by checking the metadata. You should check to make sure the resolution is good for what you want to do.

3. DOQs are images that have been corrected for distortions due to camera tilt, terrain displacement, and other factors.

4. The national elevation dataset is a seamless high resolution DEM of the United States produced by the USGS. This information is derived from both ground and aerial surveys. The National Hydrologic Dataset (NHD) compines data from USGS digital line graph data and the US EPA river “reach” data to create digital spatial data about surface waters including rivers, streams, canals, ditches, lakes, ponds springs, and wells. NHD data come from both ground and aerial surveys. National wetlands inventory (NwI) data are produced through a combination of land and aerial surveys as well This data is produced by the US Fish and Wildlife Service and they portray the characteristics of wetlands, including open water.

5. DEMs, or digital elevation maps, are data models in a raster format that provide elevation data. DEMs can be low or high resolution. DEMs make up the NED, national elevation dataset. The NED provides the highest-resolution DEM data for the United States and this data is seamless.

II. Answers to lab exercise

2a. The feature datasets are called “basemap” and “hydrology”

2b. The feature classes are NHDFlowline, NHDPoint, NHD Waterbody, and Watersheds

2c. NHDFlowline: line, NHDPoint: point, NHD Waterbody: polygon, and Watersheds: polygon

3a. Vector layer (metadata: description: expand data storage and access information: type of data)

3b. vector layer feature class, specifically a shapefile

3c. Yes

3d. Vector feature class

3e. Yes

3f. 3 keywords: hydrography, stream/river, and lake/pond

3g. Earth Science Information center, USGS

4. yes

6b. 34118-B5

7. black and white

8.

10.

12a. The lacount_lu01.shp layer was grayed out and had a red exclamation point next to it because it couldn’t find the data source

12b. To redirect the layer to the new data source name I right-clicked on the layer and selected properties. Then, I selected the source tab, set data source, and finally the lab 5 data folder and the correct data source.

Lab 4: Map Projections

The Importance of Map Projections

If you imagine peeling an orange and attempting to lay the orange peel flat on a table, you can see that it cannot lay flat without breaking and tearing. The same is true of the earth’s surface because it can’t transform from a sphere to a plane. Thus, it’s impossible to make a completely accurate representation of the earth’s surface on a flat map. A map projection is a solution to this problem. It is a two-dimensional representation of the coordinate locations from the earth’s curved surface. Map projections are necessary in order to create maps. One way to picture the creation of a map projection is that it is formed as if someone is shining a light through the projection source (i.e. a globe) and the image that is projected on the flat surface is the map projection. The flat surface that the image is projected onto is called a developable surface. This surface is then “unrolled” in order to create flat map. There are many types of map projections, but the most useful map projections for GIS are based on developable surfaces. Some examples of the most commonly used developable surfaces are cones, cylinders, and planes. Names of map projections correspond to these developable surfaces, for instance, a map projection based on a cone developable surface is called conic. Instead of using a light to create a projection, cartographers use mathematical formulas to create accurate map projections.
Map projections inevitably become distorted because they attempt to take a curved surface and make it into a flat surface. For instance, one part of a map can be squished into a smaller area while the other part of a map is expanded. Therefore, if you were to measure the distance between two points on a map distorted in this fashion the distance may be smaller or greater than the actual distance. One way to measure the amount of this distortion is by using the great circle distance. The great circle distance is the distance between two points on the earth’s surface measured along the curved surface of the earth. This is then compared to the grid distance, which is the distance between two points measured along the projected surface. The difference between the great circle distance and the grid distance represents the distortion of the distance caused by the map projection. Factors that influence the distortion of the map projection include: the location of the projection source, the shape of the surface onto which we are projecting, and the location of the projection surface in relation to the globe. However, the significance of map projections is that they aim to reduce this distortion to the extent that this is possible. Map projection aim to preserve, shape (conformality), direction, distance (equidistant), and area (equivalence) without distortion. A map projection that attempts to maintain shape and direction is called a conformal map projection. Map projections that preserve area are called equivalent, or equal area, projections. Map projection that preserve distance over a short area but do not preserve area or shape are called compromise or equidistant projections. Finally, map projections in which all properties are distorted, but the map looks nice, are called perspective projections.
The map projections I used to represent conformal map projections were the Mercator and Gall Stereographic map projections. The Mercator map projection is famous for distoring the size and shape of large objects near the poles. Thus, Greenland appears just as large as Africa, even though Africa is 14 times the size of Greenland. Similarly, the Gall Stereographic conformal map projection that I used also has an exaggerated scale closer to the poles, but to a lesser extent than the Mercator projection. The stereographic projection which is the conformal azimuthal projection, stretches the northern and southern parts of the map horizontally. The Mercator projection expands the distance between Baghdad, Iraq, and Washington,D.C. in comparison to the Gall Streographic projection. The Gall stereographic projection compresses the distance between these cities. The equidistant world map projections I used were the sinusoidal projection and the plate carree projection. The sinusoidal projection is a pseudocylindrical map projection that distorts the distance on the map between meridians such that the distances are smaller, however it preserves distances along parallels. The Plate carree equirectangular projection is centered at the equator and all parallels are equally spaced, as are all meridians. Since the sinusoidal projection compresses the distances between the meridians, the distance between Baghdad, Iraq, and Washington, D.C., appears smaller for the sinusoidal projection in comparison to the plate carree projection. The plate carree projection has equally spaced meridians and thus the distance between the selected cities is expanded. Finally, I used the mollweide and bonne map projections as the equal area map projections. The Mollweide projection is a pseudocylindrical map projection that compresses the parallels near the poles, but the other meridians are equally spaced closer to the equator. The Bonne projection is a pseudoconical equal area map projection in which the scale is only true along the parallels of latitude. From the map we can see that the distance between Washington, D.C. to Baghdad, Iraq, is more compressed on the Bonne projection. Since the Mollweide projection is more equally spaced closer to the equator the true distance between the selected cities is more accurately represented by the Mollweide projection.

Coordinate Systems and Projection Worksheet
1. What is an ellipsoid? How does an ellipsoid differ from a sphere?
It is the best fit for a particular location that approximates the surface of the earth. An ellipsoid is a mathematical surface based off of a semi-major and semi-minor axis. There are many ellipsoids approximating the earth’s surface that vary in shape because of geoidal variation in the earth’s surface. An ellipsoid is different from a sphere because an ellipsoid is a sphere that is flattened at the poles.

2. What is the imaginary network of intersecting latitude and longitude lines on the earth's surface called?
Graticule

3. How does the magnetic north differ from the geographic North Pole?
Magnetic north is where a compass points to and geographic North Pole is located at one of the poles of the earth’s axis of rotation. The difference between these north points is called magnetic declination.

4. Why are datums important? Briefly describe how datums are developed.
Datums are important because they provide a frame of reference for the ellipsoid models. Datums consist of two components, a specified ellipsoid and a set of surveyed coordinate locations specifying positions on the earth’s surface. To develop a datum one must painstakingly survey an area using the best methods and equipment. Datum points were first determined using celestial measurements. In more recent years, datum points are now determine by sophisticated electronic and satellite-based surveying systems.

5. What is a map projection?
A map projection is a 2-dimensional representation of the coordinate locations from the earth’s curved surface. Projections attempt to preserve, shape, direction, distance, and area without distortion.

6. What is a developable surface?
The flat surface that the image is projected onto is called a developable surface. This surface is then “unrolled” in order to create flat map. There are many types of map projections, but the most useful map projections for GIS are based on developable surfaces. Some examples of the most commonly used developable surfaces are cones, cylinders, and planes. Names of map projections correspond to these developable surfaces, for instance, a map projection based on a cone developable surface is called conic.

7. Which lines on the graticule run north-south, converge at the poles, and mark angular distance east and west of the prime meridian?
a. Lines of longitude

8. Which of the following ellipsoids is now regarded as the best model of the earth for the region of North America?
a. Clarke 1866

9. Which well known coordinate system would be appropriate to use for developing and analyzing spatial data when mapping counties or larger areas? Why?
State plane coordinate system projection. The state planes system limits projection errors by dividing states into zones. The zones are specified such that any projection distortions are kept below 1 in 10,000. This system reduces distortion that increases over larer distances.

10. What is a great circle distance?
The great circle distance is the distance between two points on the earth’s surface measured along the curved surface of the earth. This is then compared to the grid distance, which is the distance between two points measured along the projected surface. The difference between the great circle distance and the grid distance represents the distortion of the distance caused by the map projection.

Lab 3: Data Formats and Models

1. A data model is a model constructed after abstraction decisions have been made. It is a model that describes and illustrates features of the actual world in a digital form. The two most commonly used data models are vector and raster data models. Vector data models are called discrete data models, they have distinct boundaries and are not continuous. Vector data models are made up of points, lines, and polygons. There are two types of vector data models: spaghetti and topological models. Raster data models are called pixelized data models that define features as a set of cells in a grid. Raster data models are good for processing multiple layers and probabilistic modeling.

2. Topology is the process of establishing proper relationships between the lines, points, and polygons that have been drawn. Topology is important because it ensures proper relationships between features such that all features on the map have the correct connectivity, area definitions, and contiguity. Once these relationships are properly established, the model can be bent and still have the same connectivity and adjacency. Additionally, topology is important because it results in more accurate analyses due to a better resolution of linear and point features.

3. The best type of data model for representing hillside slope would be a raster data model using continuous data. The data model would be able to show a gradual change in elevation between the grid cells. A vector data model would make it seem that the hill is made up of discrete steps of different elevations because there are discrete boundaries in vector data models. Additionally, it is more efficient to use a raster data model for elevation.

4. If the grid cell is very large then it has less resolution. For instance, a 30 m grid cell will not very accurately represent a 15-m wide river because it will double the size of the river. To avoid this you could either use a vector data model, or use small grid cells in order to increase the resolution of the raster data model.

5. Four types of attribute data include: nominal, ordinal, interval, and ratio. Norminal attribute data is categorical data, such as the name of a city. Ordinal attribute data is ranked data, for instance the rank of cities in terms of cleanliness. Interval data is data that is related to an arbitrarily chosen zero value, such as temperature in Celsius. Finally, ratio data is all other types of quantitative data with reference to an absolute zero, such as elevation data.

6. Two types of vector data file formats are shapefile (.shp, .shx) and interchange files (.eOO).

7. Two types of raster data file formats are Imagine (.img) and GeoTIFF (.TIF) files.

8. C. select features

9. The World.mdb geodatabase contains four feature classes

10. The names of the feature classes are cities, countries, disapp_area, and world30

11. The layer file called flight_diver ge depends on a spatial dataset called the flight_diverge shapefile.

12. One way to add data to an ArcMap document is to drag the file from the ArcCatalog view to the ArcMap display window and release the mouse button. Another way is to go to the file menu and select add data from the drop down menu (or select the add data button on the toolbar).

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Lab 2: ArcGIS and ArcMap

1. ArcMap, ArcCatalog, and ArcToolbox are the software products included in ArcGIS and we will be using ArcMap the most.

2. It would be better to create, symbolize, and analyze maps in data view. The layout view should only be used for organizing maps to be printed.

3. One way to obtain help is to click on the “what’s this” tool on the toolbar (or you can get to it through the help menu) and place the pointer over an object that you have a question about. Another way to get help is to go to the help menu and click on “ArcGIS Desktop Help” or press F1.

4. Attributes are linked to geographic features through an attribute table. You can get to the attribute table by right-clicking on a layer in the TOC menu and selecting open attribute table.

5. The file called a map document, “.mxd”

6. One way to zoom in is to click on the magnifying glass on the tools toolbar and then use the magnifying glass to click on a part of the map that you want to zoom in on. Another way is to use the magnifying glass to select a box that you want to zoom in on.

7. Three operation available include: label features, open attribute table, and zoom to layer.

8. This means that the data for that layer is not able to be viewed because the scale of the map is not correct. You would need to either zoom in or zoom out in order to view the grayed out layer.

9. A small scale map is, for example, a map of the United States in which the scale could conceivably be 1: 500,000. This is called a small scale map because the fraction 1/500,000 is very small. An example of a large scale map is a map of the CSUN campus, which might have a scale of 1:10 (a much larger fraction).

10. A feature is a geographic object in a layer. Features have a variety of shapes (polygon, line, or point) and sizes. Also, features are linked to information through an attribute table. A surface is a single, continuous geographic expanse, for example the ocean is represented by a surface. A surface is made up of different values for each location on the earth’s surface and is generally represented by a raster.

11. Create project criteria

12. The minimum elevation was at New Orleans and it was 0ft above sea level. The maximum elevation was at Tuscon and it was 1045 ft above sea level. In order to find these numbers I opened the attribute table for cities and organized the table by elevation.

13. The distance from Dakar to Assab is approximately 4,172 miles. You can determine the approximate distance by using the measure tool. I suppose you could also open the attribute table for flight path and sum the distances between each of the cities from Dakar to Assab. However, that would not give a very accurate measurement of the distance.

14. You could right click on the cities layer and select label features. This would label all of the cities shown on the map. Or, you could use the identify tool and click on a city to find out the name of the city.

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