Metal Detecting and Google Earth Image Overlay


Randy (randy)

I found myself headed for an unfamiliar part of the world, and I wanted to do a little dirt fishing while I was there. I pulled out some maps, found a park I had never heard of, and decided this place looks as good as any.

But, where in this large park should I hunt? Ballfield? Tot lots? Open fields? Trails and woods? Who knows?

Modern View of Park

I'm interested in attempting to find older silver coins, and this article is a tutorial on using image overlay in Google Earth, along with old aerial photos, in an attempt to increase one's odds of success. The general idea is to find areas on the old aerial photos where people may have gathered but that are no longer obvious today, overlay the old photo onto the modern photo, and then detect that part of the park.

Google Earth

Google Earth is an awesome package that gives you a modern aerial view. It can be downloaded here. It's pretty easy to use -- navigate to the aerial view of the park to detect, and zoom in to the level of detail so you can see parts of the park that look good. The above shot of the park is from Google Earth, and it is our starting point for the tutorial. We are going to use image overlay to try to find a good place to detect.

Historical Aerial Photos or Old Maps

Google Earth's aerial imagery only goes back to 1994. The goal here is to go back as far as possible. If you are looking for silver coins, you obviously have to go back to at least 1964.

Fortunately, for Pennsylvania locations like this example, there are three sets of aerial photos available, covering the entire state. One set is from 1937-1942, one is from 1957-1962, and one is from 1967-1972. I don't know what is available for other states, but I understand Historic Aerials has a few. Poke around the web. If you can't find aerial photos, antique maps that show the location of old buildings and so forth will work just as well for the purpose of this tutorial, so long as they are about the same scale as the modern image.

Old View of Park

In any case, find the old aerial or map of the same area that you wish to overlay. For Pennsylvania, I use Penn Pilot, and for this example I used a aerial from 1958. That should give us a decent chance to find some silver coins if we can find a good area on the photo that isn't obvious today, and thus not already hunted to death.

This step can be surprisingly tricky. It can be difficult to find the same place on the historical aerial as on the modern view. The trick is to look for things that haven't changed, like obvious lakes, rivers, railroads, and certain roads. Be careful, however, as roads sometimes are re-routed. Download the historical aerial into a photo editing program like Photoshop, and crop it to about the same size and area as the area of interest, as in the example below. It doesn't have to be exact, but the closer, the better.

Find an Area to Hunt

You can see in the historical some of the features that were there 50 years ago, and try to make a guess as to where people would have been. I can think of a few ideas. You can also see that the modern ballfield and some parts to the west seemed less likely to have traffic back then, and thus less likely to have older coins.

Old View of Park

The feature that leapt out at me was the old ballfield, quite far from the modern one, indicated below, and not visible on the modern aerial at all. Experience tells us that people are always losing stuff at ballfields; perhaps it was the same back then. Experience also seems to tell me that the best place to hunt ballfields is along the first base line. Since finding old deep silver requires taking a smallish area and really concentrating on it slowly, for this example we will try to overlay the old ballfield on the modern park, and hunt the first base line, and then move to other parts of the infield.

Obviously, we could take what we have now, and just go to that area of the park and start swinging. But, the point of this article is Google Earth's image overlay, so we'll go through the exercise, because I think it is cool, and it does have some benefits. Also, in this example, part of the path near that old ballfield has been rerouted, and that is not obvious. Turns out that the location of the old ballfield is farther away from the modern path than it appears. I did this at another location without the benefit of image overlay, and missed it by 20 feet, and never realized it until I went through the exercise. It's also nice to see the new path vs the old path, as right along the side of old paths is another potential hunting idea.

Setting Up an Image Overlay in Google Earth

Google Earth Toolbar

First, make sure the map is oriented directly north (or whatever direction your historical is oriented), using the widget in the upper right of Google Earth.

Then click on Image Overlay on the toolbar.

Google Earth Dialog

The image overlay editor will come up, which is a series of green lines and shapes (explained shortly), as well as a dialog. In the dialog, browse to the location of the historical aerial or old map from above, which will bring up the historical image into Google Earth (as explained below, you can skip the browse step for now if you have a pretty good idea of the boundaries of the historical photo).

Modern View of Park

You use the green lines in the editor to line the historical image up exactly with modern image in Google Earth. Use the corner and T-shaped pieces on the edges to stretch or shrink the area. Use the plus in the middle to move the whole thing. Use the diamond on the west side to rotate the image.

You can do this either with or without the historical image loaded via the dialog. (If you want to do it without it being loaded, and you already browsed to the location, simply slide the "Transparency" slider all the way to the left).

The idea is to get it as exact as possible, and unless you managed to get the thing the exact size when you created it, you will probably have to stretch and shrink a little bit. Use the permanent features in the terrain to fit it exactly; in this case, use the river to the north, and the bounding roads. You know these are more likely to not have changed -- you can't rely on park features as much, as in this case, the one path has moved a bit, and a building has been torn down.

Saving and Hunting

Google Library

Once you are satisfied that it is lined up as close as possible, enter a name for this overlay in the dialog, and click "OK". This will now be a part of your Google Earth database for future reference, as you can see on the right.

I think this is really cool, and if you are like me, you will want to come up with a systematic way to name these. You enable/disable them on the map with a checkmark, and you can bring back its dialog (mainly to change the transparency), by Right Click -> Properties on the particular image overlay you are interested in (in this case, "MyPark").

By sliding the transparency slider back and forth, we get an idea of exactly where the infield of the old ballfield was, and can mark that on the modern map, and go out and find some silver. The animation is really cool in the actual Google Earth application. Try it!.

Google Animated So How Did It Go?

Well, it went ok. Found one silver and a handful of wheaties along the infield (three on the first base line, one near home plate). All were deep, 6-8 inches. As a test, I hunted other areas of the park for about the same amount of time, and only found one wheatie. Not very scientific, but I think it is cool. The one thing that was nice was that I had the confidence to spend time at this particular spot in the field because I knew there was something significant at that exact location in the olden days. For sure, patience and confidence are important when hunting for deep silver.

Coins Found