Chapter Six - Geometric Transformation

After using a GPS and creating a table with data paired with locations you create a map in a GIS program. Your map is of an area in space but to use your map as a layer with other maps you need to go through a reprojection to convert your tabled coordinates into a projected coordinate grid such as UTM. This is geometric transformation.

CONTROL POINTS. A set point is chosen as the control point. This point in your newly digitized map is referenced to a point in the desired coordinate system. It will be from here where your original map will be stretched to match geographically. Map error can occur if the control point is inaccurate.

Chapter 5: acquisition of GIS data

At each level of government - federal, state, regional, and local - clearinghouses exist to store GIS data for distribution. Clearinghouse data is free at the higher levels of government.

Data.gov is a geoportal meaning it includes data, news, references, a community forum, and interactive data viewer. This website connects to U.S Federal Executive Branch dataset including state, city and agency data. Geospatial Platform has been made which is like gapminder. You can combine variables to create maps. U.S Geological Survey, LiDAR, ASTER, MODIS, AVHRR, Landsat, Landsat ETM+, SRTM, Digital line graphs, National Hydorgraphy Dataset, National Aerial Photography program, are all USGS providers of data. U.S Census Bureau offers data from the TIGER/LINE, Master Address File/Topologically Intergrated Geographic Encoding and Referencing, have data on boundaries. KML is a new type of files that are used for Google Maps/Earth. Other federal clearinghouses are provided by Natural Resources Conservation Services, public data from states, metropolitans, and counties. Private companies provide data, even GIS software companies provide data like esri. 

When looking for data "pay attention to metadata and data exchange methods. Metadata provide information such as datum and coordinate system about the data, and data exchange methods allow data to be converted from one format to another.

Creation of data.

Chapter 4 - Raster Data Model

If you are going to use GIS you need to know there are fundamentally only 2 types of data. Vector data and raster data. Chapter 3 talked about vector data, and vector data is lines, points, and areas which are called polygons. Vector data represents objects that have start and end points like streets, ponds, cities, etc. Raster data on the other hand represents continuous data like elevation which needs to be recorded in a grid system where every part of the map can be represented with a different value.  

Raster is easier and better if you want to analyse your data which if you are learning about GIS that is what you want to be able to do.

So first you need to understand that a raster data map is a grid with rows and columns that make up cells. Cell one is located in the upper left. The types of numbers that get put into each cell are floating-points which is just a decimal digit, and integers which as you know means any whole number. Say you want to make map that you can analyse that has land dived up by categories. You can assign an integer to represent each category.

Raster cell's have a size which gives a map resolution. The higher the resolution the smaller the cells. A satellite image is actually a raster map. The pixels of the image represent raster cells. Raster graphs include actual pictures from satellites and even jpeg images.

An issue that exists with raster data cell's is what if in the cell there exists multiple spatial data. Like what if in the cell there was apple trees and there was wheat. For which attribute would you assign the cell? There are three solutions (they are more like compromises). First, majority rules = which ever attribute is in majority gains the cells. Second, attribute priority = if you care more about apple trees than if it is even a little in a cell that entire cell will represent apple trees. Third, boarder = just make ever cell that shares attributes a border area of no-man's land.

DOQ or digital orthophotoquad are very cool because they are aerial photos taken by a plane with a special camera on that takes so many pictures and complies them together producing a map viewing every area from a straight on 90 degrees.

DEMs or digital elevation models  are raster maps that display uniformly spaced elevation data. LiDAR is a light detection and ranging remote sensing technology that acts as a radar to see how far away things are producing a DEM.

DRGs or digital raster graphics are merely just scanned topographic maps.

Satellite images are raster maps.

The structure of raster data writing:

Raster data is written using cell-by-cell encoding. Which creates a written grid system. For multiple bands, multiple grid systems are written and compiled under band headings. Another way to write raster data is run-length encoding. This method only stores data for certain numbers in each row. By doing so files size is reduced. A final raster data structure is the quad tree. Imagine a tree representing the gird of a map. The stem stands alone representing the entire graph. From there divide the map into four. keep dividing each subgroup into 4 until you can not because all the data in a subgroup is the same. This is used for both vector and raster data.

Importing raster data:

In order to import raster data from say a DEM or satellite image you need information that tells your GIS program what you are working with. This information, found in the header file includes number of rows and columns and bands. Number of pixels. Basically information about the raster data structure, cell size, and values for no data.

Another aspect of importing is files size. Raster data files are relatively big but there are ways to reduce file size. Lossless compression allows the image to be precisely reconstructed where lossy compression allows smaller files to be made but the original image can not be regenerated. JPEG is an example of Lossy compression. JPEG's divide the image in 64 blocks. Now wavelet transformation is allowing lossless compression with lossy file sizes.

Rasterization converts vector data into raster data! This process first creates a raster grid that covers the entire vector graph. At this point all cell values are 0. The second step assigns numbers to all points, lines, and polygons. The third step fills polygons with their values.

Vectorization converts raster data into vector data! This process first creates line thinning. Second the process determines where lines begin and end. Third connects lines and shows where the errors are for correcting.

Chapter 3 - Vector Data Model

The vector model is a map of x-y coordinates depicting discrete data which is data that can be displayed as points, lines, and polygons with empty space around them. This type of data is different than continuous data which is not object defined but instead is ever flowing and changing. Elevation is mostly depicted as continuous data.

The U.S Geological survey (USGS) has established the use of a vector line to represent a river 40 feet wide or smaller. Lines are called arcs and points where arcs meet or intersect are called nodes.

Topology is a feature held by some vector models which ensures data quality and integrity by assigning an exact location to each point, line, and polygon. By doing this you can detect lines that do not meet and decide rather to make them intersect. In this way the topology feature is used for error checking and for ensuring that lines meet correctly and polygons are closed properly in vector models. For example two polygons representing two countries  would have to border each other if using topology.

TIGER or the Topologically Integrated Geographic Encoding and Referencing database is a product of the U.S. Census Bureau. The TIGER database has assigned the number 0 for points, 1 for lines, and 2 for polygons.

Not all vector models have topology though. There are three types of vector models. Coverage, Shapefile, and Geodatabase. Coverage has topology, Shapefile does not, and in Geodatabase files users can choose to have topology or not. Coverage displays topology by giving established polygons their own outlined location, defining polygon areas as distinguished lines (arcs) connecting with distinguished points (nodes), and establishing a rule that every arc has a direction and a polygon to the left and right of it. Shapefiles do not have topology, but by so doing they can be opened and used with multiple GIS software platforms. Still there are lines, points, and polygons represented in x-y coordinates but polygons sharing boarders each have their own arcs. By doing this empty space is allowed to exist. This makes files smaller as they do not hold topology data. The third vector model is called geodatabase which can turn topology on or off. In this model data is stored in a hierarchical structure which groups data in categories and subcategories. This is helpful if you want to represent an object twice with different attributes each time. Also this model allows for custom rules to be written for the model so the user can better create the graph they envision.

Regions and Subclasses of data create overlapping areas as well as joint and disjoint areas. This can not be done in shapefile or geodatabase but what can be done is creating multipart polygons which allow for polygons to have parts that join other polygons and parts that dis-join from the original, as well as polygons that overlap.

A route can be put on top of road polylines in geodatabase. This seems to break the rules but in fact is possible.

TINs are triangulated irregular networks that use vector data combined with elevation to show a 3-d map of an area. In this sense a mountain range is constructed with non-overlapping triangles. Esri has a tool that can automatically (on the fly) create a TIN when nodes,x, y, and z coordinates are known.

Chapter 2 - Turning a Globe into a 2-d map

Visualize taking the globe and placing a grid system on it, aka geographic coordinates. Doing this is termed projection. Now take this projection, this grid system and convert it into another preexisting grid system so you can share your data with data from another person's work. This is called reprojection and are both the first and second steps towards making a GIS. 

Because of this I have to understand how to make a geographic coordinate system for the globe which I can then reproject into a 2-D map of an area for my GIS. 

First, longitude, latitude, meridians (think of Greenwich England), and parallels (like the equator). The prime meridian is a line that goes north to south. From this line angles can be drawn to the left (west) and to the right (east). These angles drawn away from the prime meridian are longitude lines. Th equator is a parallel and is called 0 degrees latitude. From this line angles are drawn north and south, these lines are called latitude lines. Consider the globe as if on a x-y plane. Longitude lines are x values and latitude lines are y values. 

Conception of the globe. The globe is not a perfect sphere. Image it as being wider around the equator than it is from the north pole to the south pole. This shape I am describing is an ellipsoid. The earth is actually a very weird shape. It is in the shape of a geoid. Imagine if every GPS around the world sent in their location, and the shape of the land mass of the world was mapped out, it would show this strange shape which we call geoid.

Using a geographic coordinate system of the globe has to match up with a global model that depicts the earth as a mathematical shape. Imagine now that you have a model orange peel with a coordinate system. To take this sphere and turn it into a flat map of the orange is going to be impossible. So what you do is you figure out where you need the map to be most accurate and you try to match that section up as best you can to form a map. This is map projection. Datum is a concept that I left out of the discussion. A datum refers back to the sphere model that we made. You can match up the sphere to the location on the globe you need to be most accurate. Doing so makes your datum best suited for you but maybe not for other people.

Map projection types. A conformal projection somehow preserves angles and shapes. An equivalent projection  represents areas of the globe in relative size. An equidistant projection maintains consistency of scale along certain local lines. An azimuthal projection maintains certainlocal directions. When you make a map projection you include the type of projection in the map name.

Week One : Senior year as an undergrad begins

First day of GIS 101. The class is filled with political science majors. I am the only one in there with a health related major. GIS hasn't taken off yet in the medical field. But it will.

We are going to be using ARCGIS 10.2 I here an update is coming this spring that will change everything. I'm glad we are using ARCGIS though, because the U.S government has heavily bought into this software so every government job using GIS is going to run the latest version of arch.

Another plus about this class, is the text book is short!