The GIS 20. Gina ClemmerЧитать онлайн книгу.
place to begin. Shapefiles are layers that can be stacked on top of each other, like a layered cake, to create one composite map image. Shapefiles contain both the map and the underlying data for the map. In your GIS life, and in this book, you will need various types of shapefiles. You will also need an understanding of essential tools and key ArcMap features such as the table of contents.
Anyone who watches cooking shows knows that they breeze right over all the prep work—well, that’s not the case here. In this exercise, you’ll download shapefiles and learn key ArcMap tools.
[ You will need to download all chapter data files from esri.com/GIS20-3.
Downloading shapefiles
Shapefiles are the maps behind the maps. You need them, and you need to know a few things about them. You need to know how fresh the shapefile is (is it from 2016 or 1972?), how much it costs to get your hands on it (free or $2,000? Makes a difference), and how accurate it is (drawn by fifth-graders or NASA?). The US Census Bureau is an outstanding resource for shapefiles. It updates them every year, they cost zero dollars, and they are pretty accurate.
The US Census Bureau is the national custodian of geographic definitions of US borders. It is the agency in charge of keeping the official latitude-longitude boundaries of all states and the US as a whole.
In the following chapters, and in life in general, you will need shapefiles to create maps. Two useful shapefiles are counties and cities. You can download these two now. But first, you will need a place to save your files.
Set up a save folder
1 1.Using Windows Explorer (outside of ArcMap), create a folder on your C drive, such as C:\GIS20, where you will save files for this exercise and others. If you don’t know how to create a folder on your C drive to store files, you should ask someone. Going forward, this folder will be referred to as your “save folder.” It’s important to save on your root C drive because performing the simple task of finding your files from within ArcMap is not as easy as you might think.
Select files from the US Census Bureau website
1 1.Navigate to the US Census Bureau website, https://www.census.gov.
2 2.On the Census Bureau site, at the top of the page, click the Geography tab, and then click the Maps & Data button. Then in the right column under Geographic Data, click the TIGER Products link.
3 3.Click TIGER/Line® Shapefiles - New 2016 Shapefiles link. (These files are updated annually, so click the most current year.)
1 4.Scroll to the bottom half of the page, and click the 2016 tab.
2 5.Click the Download expandable section header, and then click the Web interface link. You’ve just entered the secret portal where current, free shapefiles live.
Download county, city, and state shapefiles
First, you must select what type of geographic file to download.
1 1.From the “Select a layer type” list, click Counties (and equivalent), and then click Submit.
1 2.On the next page, click Download national file. The County shapefile will begin to download. This file includes all counties in the US. One way or another, your mission is to get these zipped files into your save folder so they can be unzipped and you can find them again when the time comes.In Internet Explorer, you will be asked whether you want to Open, Save, or Cancel the file.
2 3.Click Save to save the zipped file to your C:\GIS20 folder.
3 4.On the Census site, after the file downloads, click the back arrow once to return to the “Select a layer type” list.
4 5.From the drop-down list, select Places, and then click Submit. “Places” is what the Census Bureau calls cities. It includes all US cities, as well as something called Census Designated Places (CDPs), which are not cities per se but highly recognized areas. The thing to know here, though, is that this file is the city file.
5 6.Select the state in which you live (you’ll use this state in many subsequent exercises), and then click Download. In this example, Alabama will be selected. Why Alabama? Because it’s first on the list. You can pick your own state.Optional: you might want to also grab the State (and equivalent) shapefile, too.If you used Internet Explorer to download files, they should already be in your save folder. If you used Chrome, you will move the files from the Downloads folder to your save folder.
6 7.Access the Downloads folder by clicking the Chrome settings in the upper-right corner, and look for three little dots stacked on top of each other. Click that button and then Downloads, and then click the “Show in folder” link. (You’ll see the zipped file there.) Right-click once on one of the files, click Cut, navigate to your save folder, and right-click Paste (or drag and drop if that’s possible).
7 8.Unzip the files (most PCs come with a built-in “unzipper”). Try right-clicking on the zipped file, then clicking Extract All, and, here’s where people get really confused, you must click the Browse button , navigate to your save folder, and save there. Unfortunately, there’s no way to unzip all the files all at once. You must unzip each file, one at a time.If you are at all confused about where you are saving your files, take a moment to understand where they are going. If you are not crystal clear on where your files are stored on your PC for these exercises, stop right now and figure it out.Once it starts to unzip, it will look as if it’s “doing something.” This activity means you’re on the right track. You should see eight files for each zipped file, with .shp and .dbf extensions, and this unzipped shapefile, my friend, is solid gold—and ready for mapping.
US CENSUS GEOGRAPHIES CAN BE CONFUSING
The US Census Bureau website allows you to select shapefiles and tabular data for many types of geography (tracts, counties, the entire nation, states, and more). Here is a quick reference to the most widely used geographies:
■Nation: this file is for the US as a whole. If you select this geography and then, for example, population as a data variable, the result will be one number, the population of the entire US.
■State: allows you to select one state, multiple states, or all states.
■County: allows you to select one county, multiple counties, or all counties for the entire US.
■Place: represents city boundaries, plus Census Designated Places.
■Census tract: tracts are the most popular subcounty geography. They are fixed in population between 1,000 and 8,000 people. Census tracts average about 4,000 people, although this amount varies widely.
Adding shapefiles to ArcMap
Open ArcMap
1 1.On the Windows Start menu, click the Windows button in the lower-left corner. In the A’s section, click ArcGIS to expand it, and from the submenu below it, click ArcMap. Ignore all those other programs—ArcMap is the mapping program, the one that makes the maps.The software will take a moment to open, which is normal.
NOTE: If you get a pop-up window about licensing, you must enter your license into ArcGIS Administrator.