GPS - Aerial Photos - Mapping

[Hotspotter]

Below is a string of emails exchanged between myself and Jason Syring, a fellow FM member, regarding mapping and overlaying GPS points on aerial photos. Because this worked for Jason, and all of the information/software is free and publicly available, I thought this might help out others trying to do the same thing.

For those of you interested in making maps of deer stand locations on the aerial photos, or marking scrapes/rubs, etc., this is a valuable tool. For the fisherpeople out there, many sunken islands, gravel bars, and mud humps show up well on the aerial photography as well. The information is fairly technical, so as always, feel free to ask any questions you may have!

Quote:

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JASON

Hey Joel –

I was doing some research on the

HSOforum

for GPS coordinates and aerial photos and saw you had lots of informative posts. I saw some of the posts last year about the NAIP photos in the MRSID format that were taken in 2003 and downloaded the Itasca county files as well as the Lizard Tech viewer that was discussed on the LMIC

HSOforum

. Last night, I also downloaded the ArcExplorer viewer because I saw that mentioned in one of your posts.

I have been able to view the photos and they are really great for checking out the area we deer hunt. What I was trying to do (and not sure it is possible) is to overlay some GPS coordinates on the aerial photos. For some reason I thought I saw somewhere that is possible. I load the MRSID file as a layer and then I have saved off a waypoint file as a shapes file (.shp) and load that as a layer as well. I can get the waypoints to show up but they do not appear on the photo – they are at a spot in the viewer that is way below the actual photo. Is it possible to “tie” the GPS points to the spot they belong on the aerial photo or am I trying to do something that is not even possible?

Sorry for the long email but I find this stuff to be really fascinating and love the potential of using for hunting. You were one of the only ones I have seen that seemed to really have a great understanding of this stuff so I thought I would see if you could answer my questions.

Thanks for your time!

Jason Syring

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Quote:

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JOEL

What you are describing is “georeferencing” the waypoints (as shapefiles) to the Aerial Photo (MrSID file). Unfortunately this involves a process called “projecting.” So you’re left with a few options:

1. Re-collect the GPS coordinates in the same coordinate system (or projection) as the Aerial Photo, which is:

Projection UTM, Zone 15, Datum Nad83, Spheroid GRS1980, Units Meters

To accomplish this, you will have to change the units/projection on your GPS unit (as close as possible to what is described above, making sure the units are in meters), and re-collect your stand locations.

Being that they’re in Itasca County, with deer season drawing near, this could be a problem!

2. Project the coordinates you do have from whatever projection they’re currently in (probably WGS84), to the UTM projection. I will probably have to do this for you, as it requires GIS software to do so. I am willing, so long as you can tell me what projection they’re in, meaning you’ll have to know something about your GPS’s current settings. I need to know everything about the current projection to georeference them, or make them “tied” together.

Let me know what you think about this, but you’re close enough to taste it! Don’t give up now! Then we can talk potential of GIS.

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Quote:

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JASON

Ok, let me make sure I understand this correctly. It sounds like it is definitely possible to do what I am trying. The problem I am having is the coordinates from the aerial photos are in a different coordinate system than what I have mine in from my GPS. How would one know what the coordinate system is for an aerial photo – did you find that in their documentation?

So, you gave me 3 options to try to resolve it.

I have really gotten into this stuff in the last year so hopefully I can get this to work. It seems like it could be pretty good stuff even if it is just for our hunting stuff! How did you get into something like this? I had never even heard of it until last year.

Thanks again so much for your help and hopefully I can get this to work!

Jason

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Quote:

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JASON

Hey Joel –

I got it to work and it is pretty cool! I tried a couple of things (#3, #1) without much success. I then decided to try and change my GPS to the UTM and Datum 83 settings and then take the waypoints off. That worked! They all came off in the UTM format and they loaded right in and showed up where I expected them. I have some very specific sights on the aerial photo that the locations are near and they all appeared correctly! It is so cool! Hard to believe that it can actually do that!

Thanks again for your help and encouragement to keep working at it!

Jason

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In the end, Jason figured it out on his own. Basically, he went into his GPS settings, and changed to the appropriate coordinate system. After re-downloading his waypoints, everything aligned.

Kudos to you Jason for figuring it out, and providing the impetus for the post. Jason has proved all you need is a handheld GPS unit, some time, and a bit of know-how to get this done!

Joel

[SteveWilson]

That's one way. There are some public domain map projection software (as well as GIS applications) available that will also perform this task. Especially usefull if you have fair amount of data to convert that isn't in your GPS unit. Myself, I use Arc/Info for the task but then again I'm a GIS bum. If you search on map projection software you'll come across a few decent links. CUNY has a nice collection of links. I used GCTP in the past and ported it to many platforms from it's original Fortran IV G.