Foundations of Geographic Information Systems Final Project
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Week6-VectorAnalysisGIS5103.pptx
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Foundations of Geographic Information Science
GIS 5103
Fall 2019 - Week Six
Vector Analysis
Objectives:
By the end of week seven you will be able to:
Describe Measurements in GIS;
Perform data queries in ArcGIS Pro
Explain techniques used in reclassification of data
Describe Buffering Functions and map overlays
Earlier in this course we covered some of the questions that can be answered by GIS. Over the past few weeks we have been discussing different types of data, data formats, data models, data creation and editing, and finding/correcting errors in data. Once we have our software, computer hardware, personnel, and data in place we can start to answer questions with our system. This week we will take a look at some of the questions a GIS can answer and also
some of the tools GIS uses to answer the questions like the ones listed below:
Where are particular features found?
What geographical patterns exist?
Where have changes occurred over a given period?
Where do certain conditions apply?
What will the spatial implications be if an organization takes a certain action?
Common questions answered by GIS
Questions for GIS Analysis
Of course “where” is the predominant question but there are some additional questions
beyond just where. Below are some examples of the questions beyond where.
Location: what is at or where is it? E.g., where is the nearest Mexican restaurant to my house?
Patterns: what spatial patterns exist? E.g., where do most of the students live in Boston?
Trends: what has changed since? Has the number of deer incidents on roads
reduced since commuters have used the deer whistles?
Conditions: where can I find something close to, within or by?
Modeling/Implications: what if? An example may include: if a new baseball park were built adjacent to Gillette stadium, how would traffic flow be affected?
Our data analysis procedures assist us in answering these questions. Let us look at them more closely now.
Making Measurements in GIS
Calculating lengths, perimeters and areas are common applications in GIS. For example, measuring the length of a road from a digital map is pretty straightforward. However, a GIS can make measurements on other types of features as well. There are a number of tools at your disposal and depending on the type of data you are using (raster or vector). The next few slides will introduce you to some of those measuring tools and techniques.
a. Length (distance); b. Perimeter; c. Area
Vector GIS measurements: (a) distance;
(b) area; and (c)perimeter can be measured as the sum of straight line lengths.
The figure to the right shows how these
quantities are calculated by the GIS, but as users it is much easier for us. All that is needed is to click on the measure tool in the toolbar of the ArcMap GIS software. The GIS does the rest.
In Vector GIS, length, perimeter and area data can be stored as attributes in a database. By storing the lengths and areas as attributes we can select features meeting a certain criteria and sum the areas or lengths. For example how many acres of wetlands do we have in a city?
Measurements in GIS
The figure above from Esri shows 3 and 5 minute drive time polygons from a downtown area. Drivetime polygons can be measured using either a straight line distance or by using road networks and traffic conditions.
Example of a Measurement
Queries in GIS
Queries are the main data retrieval operations that are useful at all stages of GIS analysis for everything from data quality checking to a final analysis.
Features may be selected by two primary methods:
Select by Location - select features based on their location relative to features in another layer. For instance, if you want to know how many homes were affected by a recent flood, you could select all the homes that fall within the flood boundary. You can use a variety of selection methods to select the point, line, or polygon features in one layer that are near or overlap the features in the same or another layer. (Esri, 2017)
Select by Attribute - Select By Attributes allows you to provide a SQL query expression that is used to
select features that match the selection criteria. (Esri, 2017).
An example procedure for selecting features by attribute might include:
A prospective buyer wants to know where all the 3 bedroom houses are for under $175,000 (apparently it’s possible, just not in Boston). The ArcGIS user can call the Selection menu in the software and then by pressing the “select by attributes” option. This will provide a new window with
fields, unique values and numerical operators within it. This window allows
the attribute table to be queried. A command line query question may then look like:
status=Y + Bedroom=3+ saleprice=$175000.
Where: status refers to house availability; 3 refers to the desired number of bedrooms; and sale price refers to… well, the sale price,and a dream sale price at that for any buyer in Boston.
Select by Attribute
Selecting by attributes involves the use of Boolean Operators (Pg. 380) in which use the conditions OR, AND, or NOT. In Esri’s ArcMap software we can use the “Select by Attributes” dialog box to build an expression. The dialog box to the right lists the layer “zebra_mussel” we want to select features from, the attribute fields “STATE” and “YEAR”. The buttons and dialog box
can be used to select all of the points from th. e feature layer “zebra_mussel” that are in the state of New York and are from 2005.
We can use this dialog box to build very complex expressions and even save the expressions for future use. The next slide will show an example of how more complex expressions can be built.
Select by Attributes
Queries Continued
This image illustrates how an attributes or non-spatial query is executed in ArcGIS. In previous chapters we had
discussed how SQL Queries can be used to select features from an attribute table. Notice the query “SELECT * FROM
Parcel_Poly_BT_2008_LOCID WHERE: BEDROOMS = 3 AND FULLBATHS= 2 AND TOTALVALUE<175,000.
Essentially, the query is telling the software to find all of the
homes in Waltham that have 2 full baths, 3 Bedrooms, that cost less than $175,000.By using operators such as “AND”, “OR”,“NOT”,etc. we can build queries based on whatever criteria we choose. I can execute this query by clicking on the “Apply” button. The parcels that are selected can then be exported to create a new layer or table. We can also modify our query to refine our selection. You will have an opportunity to gain some hands-on experience working with these queries in the co-requisite with this course; GIS5102.
An example procedure of selecting features by location might include:
An analyst wants to know where all the osprey nests are within a distance of
0.5 miles to a major highway. The ArcGIS user can call the Selection menu in the software and then by pressing the “selecting by location” tool.
This will allow the user to use the operators: “are within a distance
of” (0.5miles to freeway) and “contain” (osprey nests) in order to carry out the query.
In the following slide, an example is included of selecting particular soil types by query as part of a step to produce a soil erosion model. It is important to realize that these techniques can be used sequentially and do not have to be standalone.
Queries Continued
Example of a typical spatial query
A spatial query is used to find features that are adjacent, within a distance of, are contained by, crossed by, etc. The graphic to the right looks similar to the “Select by Attributes” dialog box on the previous slides but the main difference is that instead of using SQL queries or Boolean operators we are able to build expressions to find features that are near, within, are within a distance of, touching, or intersecting another feature.
The following set of slides illustrates how a “Spatial Query”is executed. In this following example we will be selecting all of the parcels within 300 feet of our selected parcel (highlighted in red).
Example of a typical spatial query
We can select features adjacent to or within a specified distance of a selected feature using “Select by Location”.
Example of a typical spatial query
Notice the “Select by Location” toolbox that now appears on the screen. The first dropdownbox allows me to specify which operation I would like to use. In the second box, the “Parcel_Poly” layer is checked off. By checking off the “Parcel_Poly” layer I am specifying which layer I will be querying. I could also use the select by locations dialog to query features that are in separate layers, for instance I could
also create a query that would find all of the hydrants within 300 feet of the selected property. The drop-down menu at the bottom of the dialog box allows me specify the type of location query I would like to use. In this example I will use the “are within a distance of” option.
Example of a typical spatial query
At the bottom of the dialog box I can specify a distance that I would like to use. In this case I will use 300 feet. Once I have built the select by location query I will click on the apply button and all of the parcels within 300 feet of the subject property are now highlighted in red.
Earlier in the chapter we looked at ways of finding features that are within a certain distance of a selected feature or features. This distance is commonly referred to as a buffer. Buffers are drawn based on a user specified distance and vary in shape based upon whether the feature being buffered is a point (a), a line (b), or a polygon (c). The diagram below illustrates how these shapes are drawn. If multiple features are being buffered and the buffer polygons overlap you can dissolve the buffer boundaries if you choose to create a continuous buffer polygon.
Buffering and neighborhood functions
a b
c
We discussed one way to use a buffer is to designate an area for special protection. In this example, 1000 foot buffer zones (purple) around the rail network (black) are established to define an oil spill impact area. This is what buffers around line features look like.
An Example of a Buffer
The graphic above illustrates a buffer polygon that was created to show an area 300 feet around a parcel polygon. The buffer is shown in blue. When buffering a polygon, the buffer that is created emulates the shape of the object being buffered. In this case a polygon.
An Example of a Buffer
Map Overlay: Data Integration
One of the most basic questions asked of a GIS is "What's on top of what?" For example:
What land use is on top of what soil type?
What parcels are within the 100-year floodplain? ("Within" is just another way of saying "on top of.")
What roads are within what counties?
What wells are within abandoned military bases?
To answer such questions before the days of GIS, cartographers would create maps on clear plastic sheets and overlay these sheets on a light table (remember Week 1) to create a new map of the overlaid data.
Because overlay yields such valuable information, it was paramount to the development of GIS. There are two types of overlays in GIS Vector and Raster
Map Overlay: Data Integration
An overlay operation is much more than a simple merging of line work; all the attributes of the features taking part in the overlay are carried through, as shown in the
example to the right, where parcels (polygons) and flood zones (polygons) are overlaid
(using the Union tool) to create a new polygon dataset. The parcels are split where they are crossed by the flood zone boundary, and new polygons created. The FID_flood value indicates whether polygons are outside (-1) or inside the flood zone, and all polygons retain their original land-use category values. Esri 2017.
Map Overlay: Data Integration
The total area of each land-use type in the flood zone can be calculated by selecting all polygons within the flood zone (using the Select Layer By Attribute tool, for example) and summarizing the area by land- use type (using the Frequency tool). Following is a chart illustrating the result of this calculation. (Esri 2018).
Map Overlay: Data Integration
There are a number of vector overlay tools that are common to most commercially available GIS softwares. The graphic to the right lists some of these tools. The tools you use vary based on the output you desire but they generally involve creating or updating new datasets.
You will have a chance to use some of these tools in your analysis class and also in GIS5201 – Advanced
Spatial Analysis.
We have now looked at some of the common tools and techniques used to conduct a
GIS analysis. Now that we have described some of the techniques and tools how and when do we apply them? Also, how and where do we begin? Below are five points to consider when conducting a GIS analysis.
Frame the question – Begin by defining the objectives of your analysis. What are the primary questions you are trying to answer?
Explore and prepare data – Do you have the data you need to conduct the analysis? Do you have to create new data or acquire it from another source? These are critical questions. You may not be able to acquire or develop the data you need from the sources you have access to within the time frame or cost associated with the analysis.
Choose analysis methods and tools – What tools or methods do you need to perform your analysis?
Perform the analysis – Your analysis should be conducted a number of times to make sure your
results make sense. You should have a good working knowledge of the subject matter.
Examine and refine results – Examine your results and refine your output products. Are your results repeatable? Your maps and tables should be designed to be easily understood by your
intended audience.
Common questions answered by GIS
