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I have a .jpg map of a set of U.S. counties that were affected by a tornado. The map shades where the tornado passed through, resulting in some counties only partially experiencing the tornado.

I would like to calculate the share of each county's boundary that was exposed to the tornado. Naturally, this is straight forward for the counties that were either entirely exposed or not at all exposed. However, I am not sure what to do for those that were partially exposed.

I came across this thread: How to convert image of map into vector format?

I cannot understand it, however. For starters, does Paint count as an image editor. Second, what's the deal with GRASS? Is that the best way to go? Is that in lieu of ArcGIS 10.1?

In any event, I do have access to ArcMap in ArcGIS 10.1. I am trying to follow the link: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009t000000mq000000 However, I am very lost starting around step 6. I do not know if I'm adding the .jpg correctly or assigning the points correctly.

I am just extremely lost. In the end, I would like an excel spreadsheet of the share of each county exposed to the tornado. Thank you for any help!!

To put the calculation that I'd like to do into context, here's an exmaple state-level map: http://www.marc.org/emergency/images/tornado_risk_map.jpg In this example, suppose I'd like to estimate the share of each county (or state) within the red.

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I would suggest that an Answer to the Question you reference may be your best bet if you have access to Georeferencing software (I use ArcGIS 10.1 personally). Just Georeference the image, create a shapefile, and digitise into the shapefile. –  PolyGeo Dec 6 '12 at 1:52
    
possible duplicate of How to convert image of map into vector format? –  PolyGeo Dec 6 '12 at 1:55
    
@PolyGeo How does geogreferencing work n ArcGIS 10.1? Does it matter what format the image is in to begin with? How do I create a concordance between the .jpeg map and the shapefile of counties that I have? –  user1690130 Dec 6 '12 at 1:58
1  
@JG: It does not matter which format the image is in. You can use .jpg, .tiff, and a host of other formats. Check the following link to understand georeferencing in ArcGIS: help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//… –  ujjwalesri Dec 6 '12 at 2:32
    
Thank you for the suggestions! I should add that if this can be done in a straight forward way outside of GIS, I'm all ears. –  user1690130 Dec 6 '12 at 23:21

3 Answers 3

up vote 10 down vote accepted

There is no direct route to convert an image into a shapefile format. Your jpg map has no spatial reference. You can load it into arcmap but it won't know where to put it. In order to tell arcmap where it belongs in space you have to provide geographic reference points, hence the term 'georeferencing'.

In ArcGIS this is done via the Georeferencing Toolbar, which is turned off by default. To turn it on you just right-click on a toolbar, look for Georeferencing and turn it on. Then you will use the 'Add control points' tool (first button after right of the combo box) to tell arcgis what points on the image correspond to points on a second data source that already has a spatial reference.

Here are the steps I recommend following:

  1. Open a new arcmap document.
  2. Load an existing, vector shapefile of the counties you are interested in
  3. Load the jpg of the map you want to georeference
  4. In the Table of Contents, right click on the map and select 'Zoom to Layer'
  5. Click on the 'Add Control Points' button on the georeferencing toolbar
  6. Find a point on the map that you can match to the shapefile you loaded in step 2 and click on it. I recommend finding places that are easy to identify such as intersections of major roads, sharp corners in boundaries, etc
  7. Then click on the 'Previous Extent' arrow on the toolbar to zoom back to your shapefile and move the map so that you can see the point you picked in the previous step in the shapefile.
  8. Click on the corresponding point in the shapefile. You should see your map appear in the vicinity of where you clicked. It may be very small, or it might be huge, don't worry, you'll fix that with the next step.
  9. Now you are going to repeat the last two steps using a second point on the jpg map, preferably one that is relatively far from the first one, but just go with what you can for now.
  10. Repeat the process a few times and try to get the jpg as closely matched to the shapefile as possible. You'll never get it 100 percent accurate, but you can get pretty close if you try. If something goes crazy and the map gets really distorted you can either start over entirely or delete the last control point you added using the 'View Link table' button on the georeferencing toolbar.

Then it's just a matter of either editing your existing shapefile or creating a new one using the jpg map as a background. You'll probably have to assign some attribute data such as "Damaged" or "County Name" to the polygons you draw but that is a whole other issue.

I'm sure it sounds complicated if you've never done it before but it's really pretty easy once you get your head around the concept. Just remember that there are three fundamental steps here 1) Georeference the jpg so your GIS knows where it is in space, 2) Create a new dataset from the jpg in vector format, 3) Use the attribute table from your new dataset to conduct your analysis.

Here are some links that may be helpful:

http://ocw.tufts.edu/data/54/626689.pdf

http://adielflitzow9.files.wordpress.com/2008/02/georectify.pdf

For the second phase you will have to create a new shapefile. You will use the editing tools inside arcmap to digitize the affected areas as new polygons.

1) Create a new shapefile. Open up arcCatalog and navigate to the folder you are working in. Right click and select New --> Shapefile...

2) Give your shapefile a name and under feature type select 'Polygon'

3) Click the Edit... button in the lower right and select the coordinate system you want to work with, this should probably be the same as the one used by the county base file you used to georef the map jpg.

4) Open your arcmap document with the georeferenced jpg map and add your new shapefile to the document.

5) Right click on your new shapefile in the Table of Contents, got to 'Edit Features' and select 'Start Editing'.

6) Now you can use the 'Create Features' window to digitize the affected areas on your jpg map. Make sure to save your edits often using the Editing Toolbar.

7) Once you have drawn/digitized over all the affected areas save your edits and click on the 'stop editing' button.

8) Right click on your shapefile in the table of contents and select 'Open Attribute Table'. In the window that pops up, click on the drop-down button in the upper left and select "Add Field"

9) Name your field 'Area' and select 'Double' under type.

10) You should see your new field show up as a column on the right of your attribute table, right click on its heading and select 'Calculate Geometry', choose area and the units you want, this will tell you the area of each of the polygons you digitized.

That should give you everything you need to show the affected areas on a map and calculated the total area. Again, if you need further details or more instructions these steps have been heavily documented all over the web and a search for 'arcgis create shapefile' or something similar will give you lots of results with pictures/video that will be more descriptive than my summary.


Regarding your last comment: You should now have three items in your table of contents. One should be the jpg map you georeferenced, a second should be the county map you used to georeference the jpg map, and the third should be the polygons of affected areas you just digitized.

To make a map that you can export and use as an image or figure somewhere else (ie a report or webpage) you will need to switch ArcMap over to 'Layout' view. (Go to the 'View' menu and choose 'Layout View') This is where you can add items like a legend, north arrow, and scale (via the Insert menu). Once you have things looking the way you would like them go to File, Export Map..., and choose the location, format, and resolution for the file you want to export. Then you will have an image file of the map you just laid out that you can use elsewhere.

I realize those instructions are pretty bare bones - if you need more help than that then that should really be a separate question, or the focus of some Google searching and ArcMap documentation reading on your part.

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Thank you!! That was incredibly helpful. I believe I got through step 10. However, I am not sure what level of RMS is realistic. But more so, I'm not sure where to go from here. I tried the Rectify option but to no avail. You suggest to edit the existing file or creating a new one using the .jpg map--how would I go about either. As I stated earlier, my goal is to estimate the share of the counties through which the tornado passed through, where such ground is denoted by the shaded region in the original .jpeg image. –  user1690130 Dec 8 '12 at 4:34
    
Determining the appropriate level of RMS is really up to you and the tolerance limit for your project. You will just have to decide when it's 'good enough' as there probably aren't a lot of 'best practices' documents out there to guide you in this particular project. Once you have the map in the right spot you have to switch yourself over to vector mode for the second phase of your project. I would make a new, blank, polygon shapefile and project it using the same coordinate system as your counties file. Then use arcmap to edit it and digitize the affected areas into the new shapefile. –  Kevin Dec 10 '12 at 15:15
    
Your new shapefile can then be used to depict the affected areas for each of the counties and calculate the area. Remember you are using your jpg map as a reference to create a new dataset in vector format and that is what you will use for your analysis. Once the areas you are interested in are digitized from your jpg map into a new shapefile the jpg map is no longer needed. –  Kevin Dec 10 '12 at 15:18
    
Also, you could go old-school and use a dot-grid: geog.ubc.ca/courses/geob373/labs/dot_grid_instructions.html –  Kevin Dec 10 '12 at 16:00
    
Could you please kindly walk me through the last few steps as you so excellently did the first ten? I guess starting with the projection part? So you are saying not to rectify (which isn't working anyways) but instead to project? How is that done? And how should arcmap be used to edit and digitize? How does the new shapefile depict the affected areas? –  user1690130 Dec 10 '12 at 23:05

if you want to try an opensource solution for georeferencing your image with some GCPs (Ground Control Points), you can go with this way.

firstly use your gcps.

gdal_translate -of  GTiff  -gcp 0 0 31.7431761644 35.1680410195 -gcp 3527 0 
       31.7493769674 35.1784535489 -gcp 3527 2492 31.7431011291 35.1784951643
      -gcp 0 2494 31.7431761644 35.1680410195 myImage.jpg myImage.tif

then

gdalwarp -s_srs epsg:4326 -t_srs epsg:4326  myImage.tif warped_myImage.tif

i hope it helps you...

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Thank you for taking the the time to help. Could you please elaborate in much greater detail? I hardly have a clue what any of what you wrote means or how to implement it. –  user1690130 Dec 10 '12 at 22:44

The method here works in ArcGIS10.1. Besides the jpg map, you also need another layer: a projected counties map. Steps include: 1. Georeference the jpg to the projected coordinate system of the projected counties 2. create a new vector with the same projected coordinate system in ArcCatolog 3. use editor to create polygon for the vector by taking the jpg as base map 4. use intersect in toolbox to get the common area between areas tornado affected and counties.

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By projected counties, do you mean a shapefile with the same set of county boundaries as the .jpg? How do I tell GIS that I'm only interested in the part of the .jpg that is say shaded yellow (the counties where the storm passed through) and then from there, I guess I can get the area, as long as it tells me the area of said county that is shaded. I'm not sure what you mean by ArcCatalog though. Is that an option under Tools like Analysis? And what exactly should I do in editor to create the polygon for the vector? Where do I go within editor? –  user1690130 Dec 6 '12 at 23:20
    
Yes, a polygon shapefile representing the counties of interest. If you add that to ArcMap, you can use it to reference your jpg. Adding a snapshot of your jpg may help us to answer better too. –  Baltok Dec 11 '12 at 20:27
    
@Baltok I'm not sure how to add a snapshot, but I posted a link to a similar map (although a broader geographic level). In the original post by Charlotte, I think I'm good on step 1 and am pretty sure I understand step 2 although perhaps not. Could you please walk me through the remaining steps? –  user1690130 Dec 12 '12 at 23:23
    
@Charlotte Can you please explain more about the 3rd step to use editor to create polygon for the vector? Am I copying the counties into the new shapefile or the storm region? And how would the new shapefile know about the other? –  user1690130 Dec 13 '12 at 20:14

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