Monday, November 07, 2011

How to measure areas using a free GIS System

If you want a really a quick and easy way to measure areas using an online tool, i recommend Google Planimeter. If you want a more robust solution and have the need to manage multiple fields, read on...

Measuring areas and tracking land units is a common task for environmentalists, farmers, ranchers, and land planners. This tutorial is meant to get you started in not only measuring areas, but tracking information related to fields, creeks, and any other spatial management units.

This tutorial covers how to download free imagery online, install a free GIS application, build a geographic layer (GIS layer), and measure areas. The examples given in this tutorial assumes that you live in Oregon but other sources of data in other areas can be used.

The length of time for going through this tutorial is about 2 hours and requires a good working knowledge of computers.

Step 1: Downloading Imagery

The State of Oregon has a great resource called the Oregon Imagery Explorer:

Using this tool, you can navigate to a specific address and download geo-rectified imagery. A geo-rectified image knows where it is on the earth’s surface and is important for being able to accurately measure areas. Downloading imagery from this application is easy, but you when saving your image, you must do the following:

Click on “Extract Defined Area”.
• Projection = NAD83/UTM Zone10N (western Oregon) OR substitute NAD83/Zone 11N (eastern Oregon)
• Output Format = JPEG
• Enter your name and email address and leave all other settings as they area.
• Click “Submit”

I recommend creating a directory on your hard drive to store all of the data relating to this image and subsequent layers from the following step (there should be two files that are kept together). When you download your image it should be compressed as a zip file and so you can usually just click on this and your computer will extract the files.

Step 2: Downloading and Installing QGIS

QGIS is a powerful Geographic Information System (GIS) software, equivalent to other packages that cost thousands of dollars. The drawback is that installation is a bit more difficult and documentation is not as good. You will need to follow the installation instructions at the QGIS site very carefully.

Download QGIS from . You will need to choose your operating system and follow instructions for downloading a couple of other pieces of software that must first be installed before installing QGIS.

Step 3: Setting up your QGIS Project

Create a new project and save this project in the directory you made in Step #1. I recommend saving often as occasionally this software may crash and will need to be restarted.

Add your Imagery by clicking “Layer”, then “Add Raster Layer” and pointing to the directory where you extracted your imagery. Choose the file with the *.jpg extension.

Setting the Projections:

This step is very important. Essentially, you are letting QGIS where the image that you downloaded sits on the Earth’s surface. All of your future measurements and area calculations rely on this step in order to work properly.

Click on “Settings”, then “Project Properties”, then click “Coordinate Reference System (CRS)”. In the Coordinate Reference System List, choose “NAD83 / UTM Zone 10.”, the click OK

Highlight the Layer in the view window. Then click “Layer”, then “Set CRS of layer(s)”, and choose “NAD83/UTM Zone 10” again.

Now, you are also setup to begin working with your project. I recommend saving your project at this point and pour yourself another cup of coffee.

Creating a Layer:

Supposing you are a farm owner with a number of different fields that you manage. This next step will show you how to create a digital representation of those fields in your GIS software.

Click “Layer”, “New”, then “New Shapefile Layer”.
In the dialog box, choose Type = “Polygon”
• New Attribute = “areaMeters”, decimal, width=20, precision = 4
• Click on “Add to Attributes List” (it should appear below)
• New Attribute = “areaAcres”, decimal, width = 20, precision = 4
• Click on “Add to Attributes List” (it should appear below)

Click “OK” and then save your new layer to the directory that you made.

Editing the Layer:

In the view window right click or control click on the layer name, and select “Toggle Editing”. This will allow you to edit this layer.

Click the little circle with nodes on it on the menu bar. If you hover your mouse over it, it will say “Capture Polygon”. You can draw polygons around your features with this, draw as many features as you like. Using the imagery as a backdrop you can easily trace your fields with this tool and create a separate polygon for each field.

Updating Area:

In this step, you will figure out the square meters and acreage of your polygons.

1. Right click the layer you just added, and select “open attribute table”

2. Update areaMeters
• Click on field calculator (little calculator image at bottom)
• Select “Update existing field” in the dropdown select “areaMeters”
• Under operators, click “area” (Under “Field Calculator expression”, there should appear a $area) then click OK

3. Update areaAcres
• Click on field calculator again
• Select “Update existing field”, select areaAcres
• In “Field Calculator Expression”, type “area*0.000247105381”, then click OK
• Finally, you may need to click the“areaAcres”to get the calculations to show up.

If you made it this far and everything works then congratulations! If not, don’t despair, there is plenty of documentation available online—look at the QGIS site for their documentation which will be helpful. You may also want to consider hiring a specialist to get you setup and give you a personal tutorial so you can get going.

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