As part of the Grassroots Mapping Curriculum series.

Step 3: Selecting images and preparing a map

http://publiclaboratory.org/wiki/grassroots-mapping-curriculum

3.1 - Introduction

Ok, you’ve just finished collecting some aerial images and you’re ready to make a map out of them! This is an important step because aerial photos are not maps -- that is, you can’t easily compare them with other maps or geographic data. Also, if no single image depicts your whole site, you’ll also have to combine some images to create a complete map of your site.

Resources:

• Web tutorials:
  • Grassroots Mapping online curriculum
• Videos:
• Examples:
• Ask for help: mailing list, team@ address, phone #?

• Advanced topics:

  • Multispectral imaging

• Timeline of a project: how does this step fit into the overall process of map making?

  • Preparation (1-2 weeks minimum): A mapping project starts with a discussion of what you want to make a map of, why, and for whom. Consider who the stakeholders are and what new information you’re trying to provide. It also includes a site survey - visit the site, if you don’t live there, talk to folks who do, check wind and weather patterns, look for good sites to fly from. Look it up on Google Earth and use the Historical Imagery slider. Procure materials and assemble a balloon or kite kit.
  • Attempt mapping (1-3 days minimum): ... Be sure to collect careful field notes and photos, and take advantage of any spare time to talk about why you’re mapping. This can be a good opportunity to do some interviews or videos. Be sure to save a complete copy of the map data, field notes, photos, and interviews.
  • Map preparation (2-5 days: what this document describes in detail): Sort through images for the best ones manually or with MapMill.org, and find old reference imagery for the subject area to use as a base layer. Sorting online with MapMill.org provides a great opportunity to reach out to a larger online audience in a participatory way about the issues that concern you. Use Knitter or Photoshop to place images over the base layer and align them. Slowly add and refine your map; export it in an appropriate format.
  • Publication and outreach (1-3 weeks minimum): Collaborate with local residents and other interested parties to examine, interpret and comment on the map you’ve created. Prepare the map for publication online or in print, and be sure to contextualize the new information you’ve gathered; if you’re printing the map, consider publishing interviews, opinions, and analysis on the reverse side. A printed map is a great way to communicate with a broader community about the issues you are highlighting in your project; make it an invitation for commentary and discussion.

3.2 - MapMill: Selecting aerial photographs for making aerial image maps

• Background: After completing an aerial image acquisition flight the raw images must be heavily sorted in order to begin making a map from the separate images. Ideal mapping images are aimed straight down, not blurry, and not too low to the ground.
• Sorting System: MapMill uses a rank sorting system where images are classified as (1) really good, (2) just ok, or (3) not useful. The values are totaled within each flight or “set”. The sorting of the values will help the user make the actual map with the next tool in the map production process: MapKnitter.
• Example: of voting on images with screen shots (or video)

3.3 - MapKnitter: Map stitching

• Background: Known as “orthorectification” or “georectification” to geographers, this step covers the process of figuring out where images can be placed on an existing map, and where they can be combined, or “stitched” together. You are likely to have many images of overlapping or identical areas.
• Example Project
  • Important. screen shot/ video walk through of a knitter project
• Selecting background/reference imagery
  • Sources: Google Maps (check license at bottom right), Public Domain: NOAA/USGS,
    • Option: collect your own from an airplane, if you happen to fly over on a commercial flight
    • Option: use Ground Control Points (GCPs) -- advanced topic
  • Knitter: already set up
  • Photoshop: screenshot from Google Maps: beware of projection
• Placing your first image
• As you continue adding images: Keep looking back at your base image; if you rely too much on overlapping your new photos, you can “drift” from the reference
• When you’re done, or when to stop:
  • How obsessive are you?
  • 10-30 images
• Exporting: resolution, area?
• Advanced topics:
  • Distorting vs. “warping”
  • Motion blur, lens blur
  • Exposure: try “auto levels” or equiv.

3.4 - Next Steps

• Archiving
  • The PLOTS Archive
  • Data storage
• Publishing
  • Print Map Production:
  • Online Map Production: