Photographs to Maps Using Aerial Photographs to Create Land Cover Maps

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Francine O’Neal’
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In addition, a map showing general patterns may be easier to read and understand than a photograph.

1 Aerial photographs are an important source of information for maps, especially land cover and land use maps. Using ArcView, a map composed of points, lines, and areas (vector data) can be constructed from an aerial photograph. Vector data may be preferable to images for some purposes, (e.g., measuring the area of land used for parks). In addition, a map showing general patterns may be easier to read and understand than a photograph. The images at right show a photograph and a map constructed from it: Aerial photographs are available in several formats. For this exercise, a photograph that already is in a digital format is preferred. The USGS produces digital photographs called Digital Orthophoto Quadrangles (DOQs), and these instructions assume that you are using a DOQ. However, if you have a paper print of a photograph and a scanner, you can create a digital photograph. DOQs are available from the USGS, although they are not available for all areas. DOQs are distributed in a number of delivery options, including FTP from the internet, compact discs, and 8mm tapes. For more information about obtaining DOQs contact the USGS. This document illustrates how to create a land use map from an aerial photograph. It assumes the reader is familiar with basic ArcView procedures such as adding themes or creating a thematic map, and with intermediate ArcView procedures such as creating and editing spatial data. Essentially, this process consists of four steps: Define the study area and acquire the necessary photograph(s) Determine the format and resolution of the desired output Interpret the photograph and create the vector data Ground truth the results

2 Step 1 - Define the study area and determine its location on the photograph Decide what area will be mapped (a neighborhood, a forest, a township) and find that area on the photograph. Reference the photograph to the real world. Use other types of information to orient yourself (e.g., street maps, USGS topographic maps). If you are using a DOQ, the photo has geo-referencing information that can be used to reference the vector data to the correct real world locations. Each DOQ is accompanied by a "header" file that contains the information needed to geo-reference the photograph. A sample ArcView extension called the "USGS DOQ Reader" is available (from which can register the photograph automatically. Alternatively, you can create a world file using the information found in the header file. If you are using a scan of a printed photo, find some clearly identifiable features on the photo and find those same features on a map. Using the map, determine coordinates of control points and use the coordinates to geo-reference the photograph. Using ArcView, set the View Properties to reflect the coordinate system and map projection of the photo. How?

3 Setting the View Properties and Map Projection From the View menu, choose Properties Select the Map Units and the Distance Units Click the Projection button and select the map projection Back to Step 1 - Define Study Area Continue to Step 2 - Determine Format and Resolution of Output

Step 2 - Determine the format and resolution of the desired output Before you begin interpreting the photograph, decide what the finished product should look like. What categories should be included?

Do you want to distinguish between "low density residential" (houses) and "high density residential" (apartments) or is "Residential" sufficient?

Think about the categories that will be useful before you start creating data. You can always adjust the categories if need be, but a little planning before you begin can save much time later.

For example, you may want to identify Residential, Commercial, Park and Agricultural (see samples at right). How much detail is desired in the finished product?

4 Step 2 - Determine the format and resolution of the desired output Before you begin interpreting the photograph, decide what the finished product should look like. What categories should be included? How much detail is desired? What attributes will be recorded? What categories should be included? Do you want to distinguish between "low density residential" (houses) and "high density residential" (apartments) or is "Residential" sufficient? Is it useful to show the difference between "high density commercial" (downtown) and "low density commercial" (strip malls)? Think about the categories that will be useful before you start creating data. You can always adjust the categories if need be, but a little planning before you begin can save much time later. Make a list of the categories you will use and spend some time looking at the photograph for examples of each. For example, you may want to identify Residential, Commercial, Park and Agricultural (see samples at right). How much detail is desired in the finished product? While it may be possible to identify small features on the photograph, it may not be useful to include every detail in the output. For example, do you want to show the outline of each building or just differentiate areas covered with buildings from those without any buildings? Reducing the amount of detail often makes it easier to see general patterns. Determine a threshold for the smallest feature that will be recorded, such as one acre or one city block. Residential Commercial Park Agricultural What attributes will be recorded? At a minimum you will want to record an identification code and the land cover/use. You also may want to record the area of features (in square feet or square meters) or the names of features. For this example, three attributes will be recorded: an ID code, a description of the land cover/use, and the area of each parcel in square meters.

5 Step 3 - Interpret the photograph and create the vector data Create a new theme and establish the working environment Create a new theme, selecting Polygon as the Feature Type. Set the Snapping environment for the new theme (see ArcView's on-line help for more information). The snapping environment controls how feature meet and align as you create them. If desired, change the symbol to a semi-transparent pattern (e.g., hatched rather than solid fill). This will allow you to see the photograph and the vector data you are creating simultaneously. Open the theme table and create new fields for the attributes. How? Delineate areas of similar land cover/land use using the drawing tools and record their attributes Use the polygon drawing tool to draw the first shape. Draw a polygon around an area with a uniform land cover. Assign the new polygon an ID code and record its attributes (e.g., description of land use/cover, area in square meters. How do I measure the area? ) If the next polygon to be added is adjacent to the first, use the auto-complete to add the next shape; this eliminates "double-tracing" shared boundaries. Continue adding polygons and recording attributes until the entire study area is covered. Alternatively, you could add all the features first and then enter their attributes in the theme table afterwards. Use whichever methods works best for you. View a sequence of images illustrating this process

String for text information Use Date for dates Width is total width, including decimal places

6 Adding fields to the theme table From the Edit menu, choose Add_Field Define the characteristics of the field Field Names have a limit of ten characters Use Number for numeric information Use String for text information Use Date for dates Width is total width, including decimal places Back to Step 3 - Interpret the Photograph and Create Vector Data Continue to Step 4 - Ground Truth Results

Measuring the area of polygons As you draw a new feature, ArcView calculates its area and displays the value in the Status Bar. Note that value so you can enter it in theme table.

7 Measuring the area of polygons As you draw a new feature, ArcView calculates its area and displays the value in the Status Bar. Note that value so you can enter it in theme table. The units of measurement that will be displayed correspond to the Distance Units selected in Step1 (setting the View Properties). Alternatively, you could have ArcView calculate the area of each feature using the Calculate button but it would return a value in decimal degrees. Because the area of a one degree by one degree square is not constant over the earth's surface, this is not a very meaningful unit of measure. Use square meters or square feet instead. Back to Step 3 - Interpret Photograph and Create Vector Data Continue to Step 4 - Ground Truth Results

8 View containing an aerial photograph

9 First polygon added to the new theme

10 Attributes of first polygon added to the theme table

11 Several polygons added to the new theme

12 Map of land uses created from aerial photograph Back to Step 3 - Interpret Photograph and Create Vector Data Continue to Step 4 - Ground Truth Results

13 Step 4 - Ground truth results For a sample of areas, evaluate your results by visiting the actual location or consulting other sources such as topographic maps, street maps, or other large scale maps. For any areas that couldn t be determined from the photo, visit the actual location to evaluate the land cover/land use, or, if that is not possible, use other sources such as topographic maps, street maps, or other large scale maps. Congratulations! You have converted an image to a map.

Lecture 9: Reference Maps & Aerial Photography

Lecture 9: Reference Maps & Aerial Photography I. Overview of Reference and Topographic Maps There are two basic types of maps? Reference Maps - General purpose maps & Thematic Maps - maps made for a specific