DEM -> contours -> OCAD

Using a USGS digital elevation model (DEM) to generate contours for an orienteering map might be described as "garbage in, garbage out." The USGS DEM is crude at best, at least from the viewpoint of an orienteering map.

But what this technique does offer is the ability to generate contours at a different interval than a USGS quadrangle map. For the Chattahoochee Bend map, I need countours at a 3 meter interval. The USGS quad has 20 foot contours. The object is to take the DEM, run it through some utility programs, and generate contours at a 3 meter interval that I can use a a crude base map.

Every contour must be redrawn during field checking to more accurately match the terrain, but there is an advantage to having contours at the correct interval to start from.

So I got a DEM of the area, for free, from the Georgia GIS Data Clearinghouse. This came in NAD27, which is what I was using with the DOQQ and NAIP photographs, so my OCAD map is already using those coordinates. If I were using some other coordinate wywtem I would have to convert so hat all my data sources are on the same page of the hymnal.

I use the GDAL utilities to do coordinate transformations and to generate the contours. These are command-line programs that are available for almost every operating system. I'm using a mac - not that it matters. The program to generate contours from a DEM is gdal_contour. The command looks like this:

gdal_contour -a contour -i 3 e3384d8.dem whitesburg.shp

The default output is a shape file. Unfortunately, the GDAL utilities don't seem to be able to generate DXF files, which is what OCAD needs. So I had to find a program on the net to do the conversion. I used Arcv2cad, which had a demo version. The dxf file is then imported into OCAD and the contours converted to a symbol. I actually copied the contour symbol and changed the color to gray.

So how does the output look? And how does it compare to the USGS quad contours? Here's an excerpt of the map:

[caption id="attachment_26" align="alignnone" width="341" caption="USGS contours and DEM contours"][/caption]

The USGS contours are in brown, and are at a 20 foot interval. The DEM contours are in gray at a 3 meter interval. So there should be about twice as many DEM contours. Note also that the DEM contours are points connected by straight line segements, and the USGS contours, which are taken from the DRG, are smooth lines.

This excerpt is about 850 meters east to west and 600 meters north to south, so you can see that there is not a lot of elevation chage on this map.

So is this useful? I would like to have both in the field, so that I can contrast and compare. The DEM gives me a clue where the extra contour line should go.

This would not be useful in a map with relief detail that could be obtained with photogrammetry. Photogrammetry would be a vast improvement when there is more relief and there is detail to be discovered. But this is such a "gentle" area that the photogrammetry is not going to reveal much, and comprehensive field checking is needed anyway. It is during the field checking that the detail will be added and the contours smoothed.

NAIP photographs

I am wrestling with the decision of whether to have photogrammetry done or not. It's the traditional way to go. But this map lacks intricate contour detail, so it is debatable that photogrammetry will do any good.

What is needed are recent aerial photographs. I found a new program from the department of Agriculture that is trying to take recent photographs, and I bought two photos that cover most of the park from 2007.

The first example below is a portion of the barerock area. There is a swath of rock features that run across the map. The second is of the northern tip of the horseshoe bend, showing some of the old fields and the new pine plantations. The quality and size of the photo excerpts is reduced, of course.

[caption id="attachment_23" align="alignnone" width="256" caption="Bare rock at Chattahoochee Bend"][/caption]

[caption id="attachment_22" align="alignnone" width="256" caption="Horseshoe Bend"][/caption]

The plan is to load these into Ocad as templates, and print them at 1:7500 with grid lines and other info, then take them out into the field and use them for field checking.

Jan 2009 Update

Spoke with my contact at the DNR this morning to get an update on the park. Last year he had told me to call back in January. Here is what I learned:

- Big surprise - development money has been released, despite the budget problems. They are already widening and paving the road leading into the park. Planning will begin to design the facilities now. It will probably take a few more years.

- They had a lot of problems with the hunting season. He would not give me specifics except to say that the surrounding landowners filed a lot of complaints. Because of this there is a chance this will be the last hunting season, although personally I wouldn't count on it - the hunting lobby is pretty strong.

- Logging will continue a little longer. They are thinning all the pine forests.

I now have permission to go out there and actually make a map, but I cannot go until after Turkey season - May 15th, and I have to call before I go and let him know I will be on the property. This is all because of the complaints they have gotten about the hunting and for liability problems. Not good news, but I have to do what the DNR says.
So I have sent an email to the board of GAOC, informing them of what I have found out and that I now have permission from the DNR to go on the property and make a map. Hopefuly we can squeeze in a meeting this weekend at the A meet and discuss it.

Talked to the DNR

Before I can do anything about making this map I need to get permission, so I talked to the DNR person who has responsibility over what will be Chat Bend park. I have worked with him before, when I made the map of FDR State Park. I'm not going to name him here, since I don't want to put him under any pressure or scrutiny. He is very helpful and always goes out of his way to assist us.

Anyway, he said the 2010 opening date is overly optimistic. As everyone knows, there are big budget problems statewide, with talk of park closures. They might open in 2010 if Coweta county supplies an operating budget for the park.

He does not want to allow anyone onto the property right now for liability reasons. Hunting is going on this year - and I really can't map during deer hunting season anyway. The decision of whether to have deer hunting next winter will be made in the Spring. This deer hunting is different than it was at Joe Kurz and Dukes Creek. In those parks the hunt was restricted to specific weekends just a couple of times a year, so I was able to work around the schedule. At Chat Bend they hunt every day.

He says we can take aerial photos, etc., now. if we want to.

The thinning of the pine forest will continue until probably January, weather permitting.

I can call him back in January and check on the status.

So this means more delay, much as I hate to say it. Seems like we have been waiting forever for this park to get off the ground. This is a unique situation too, since the land is sort of in limbo until the park actually opens. The pressure to hunt on the land until it becomes a State Park is pretty heavy too.

My suggestion is that we do nothing until January, when I will call back and get an update.

Things to bear in mind:
- wait to take any aerial photos until after the logging is completed.
- or wait to take aerial photos until we know if there will be deer hunting in the fall of 2009.
- or take no photos at all. Use the USGS contours like I did at Joe Kurz. The topography is not the same challenge as it was at Duke's Creek, and I may be able to make a good map without expensive photogrammetry.

First Visit

Chattahoochee Bend State Park has been in the works for years. I don't know all the details, but I remember first calling the DNR about the park back in 2004. Recently Laurie and I have seen more and more aricles in the Newnan paper, and Laurie went to an organizational meeting of the Friends of Chatahoochee Bend State Park group that is forming. She is a great organizer - I wish she could do all the things that I hate doing - making phone calls, finding contacts.

We finally found out that the park is moving forward, with an opening date projected for 2010. Two years - just time enough to make an orienteering map.

The last map that I made, of Duke's Creek/Smithgall Woods, took two years. Before that I mapped FDR State Park, Joe Kurz WMA, Sweetwater Creek State Park, and Cochran Mill Park. That's a lot of time on the ground making maps. And that is where the work is actually done, on the ground - not sitting in front of a computer screen.

So I made a trip out to the new park site to see what I could see. While the park is in this state of limbo, the DNR is allowing deer hunting. Hunting season starts Saturday, and the act of walking up and down and back and forth while making a map is not really compatible with deer hunting! So I make a special trip out there on a Thursday afternoon, (after a dental appointment - ouch!).

It's about twenty miles from my house, downstream on the River, in the next county south - Coweta County. It's actually right across the river from the map we have at McIntosh Reserve. McIntosh is a much smaller area though.

The first thing I noticed while driving in is that no work has been done yet on making this tract of land a park - rough gravel roads. Then I noticed recent logging activity. Not clear cuting, but thinning. The pine forests on the top of the hills was being thinned. Some of the areas seem to have been thinned some time ago, and are looking pretty good, other areas are actively being cut. I was impressed by how clean it looked. Sometimes the loggers leave huge piles of slash, but there was none of that visible.

I parked at the hunting kiosk, and was immediately mistaken for a hunter. Must be the gray beard. Since I was carrying a map (I had thrown together an improved USGS quad with the boundaries on it) I was mistaken for someone who knew something. I knew nothing, of course, except what I had read about the hunting season. I impersonated a hunter by keeping my mouth shut, signed in at the kiosk, then drove down Flat Rock Road.

I found the southern boundary, turned around and parked on the flat rock area. Looks like a miniature Arabia Mountain. Lots of little nooks and crannies that are bare rock. Should be great orienteering when mapped correctly! I proceeded down hill to the west from there, and saw plenty of boulders on that downhill, reminiscent of Cochran Mill. There's more logging in that direction, with new roads cut across the hills - too bad. Once down off the hills, the logging stopped, or maybe they just haven't gotten this far yet? Made my way down to the river for the obligatory photo looking out onto the water, then I climbed back up to check out the private property.

There is about a square kilometer of private property that is an "inholding", completely surrounded by parkland. The boundary is well marked. There is a house, or hunting shack, hiding in there.

Then I proceeded north,looking for the open fields that are marked on the USGS quad. Found plenty of overgrown farmland, and some collapsed houses and barns and old farm equipment, but no open fields. I don't think I went far enough north - the photos on google maps show some fields way up in the bend of the river. Or they may be all overgrown now...

Made my way back to the truck, Saw plenty more hunters out scouting for opening day. I pity the poor deer - saw plenty of them while out in the woods, and lots of sign.

So what did I learn on this first trip? It's typical of the other maps we have in this area, with the added unique open rock areas. There are a lot of little hills, spurs, saddles, and reentrants that should provide a good challenge too. Looks like it will make a great map, depending on what happens with the "improvements" as they construct the infrastructure for the park and how extensive the logging is. If the logging is just thinning, followed by controlled burns, it should make it better for orienteering.

Next step in the process? Obtain permission from the DNR to actually make a map before the park is open for business...