How do I use my GPS to make a map of an island

[+RiverCacher]

A conservation organization that I am on the board of was recently given a small (2 acre) island in the James River.

 

For numerous reasons, we need a map of the island that is to scale and (hopefully) would show the topography of the isle better than the USGS Topo Quad does (it shows the island as being flat, but it has topography greater than one contour line).

 

My question is how do I use my etrex venture to somehow create a map of the island. I also have an ipaq with Topo USA 4.0 that can be connected to the GPS, if that helps.

 

And yes, there will be a geocache there by December.

 

Scott

 

Lynchburg, Virginia

[+V7L]

Well, this might not be the best way but...

 

Sounds like the perimeter of the island is already well defined, if not the just walk around the island and record lat/long every few feet. Easy, huh, but you'll still have to convert Lat/Long scales to map it on graph paper, see Markwell's site

 

If your GPSr is accurate as to elevation, the divide the island into a grid (ala graph paper). The grid might represent 0.010 degrees. Go stand at each grid point (by moving 0.010 N/S or E/W) and record the elevation. Then use colored pencil on each graph point, where blue for < 10 feet, red for < 20 feet etc.

 

Hope this helps, but if you don't understand the Markwell data, this won't be much help.

 

V7L

[+JoGPS]

If you can get your hands on a Garmin Vista it has a thing that you can map out the outside of anything that you just walk around and give you the inside acre's..........JOE

[+cachew nut]

Mapsend Topo shows some contour lines on some of the islands in the James River. Which island is it? Do you have any coordinates?

[+MrGigabyte]

If you want contours, there are 3 ways to do it. All will yield nearly the same result.

 

1) Boat or canoe or walk around the island close to the shore recording the track. This is your perimeter and will be a benchmark for accuracy (assuming the river is not raging, all the track points should be reasonably close in elevation)

 

2) On the island, record waypoints at the significant "break points". These are all the high points, low points, ditches, ponds, outcrops, top of banks, bottom of banks, etc. You cannot have enough waypoints. These point are used to build a "TIN" model (Triangulated Irregular Network). The plane formed be 3 proximity waypoints should be perpendicular to the contours, if you took your waypoints in the right spots. Download your waypoints and track logs. Convert them into a delimited text file of X, Y, Z coordinates.

 

3) Find your local friendly surveyor and talk him into loading your text file into his "DTM" package (Digital Terrain Model), and hit the contour button. Done, Print. This step should take no more than 5 minutes.

 

4) Failing to locate said surveyor in item 3, buy your self DTM program. They can be had for under a hundred dollars and many integrate seemlessly with AutoCad (SoftDesk, EaglePoint etc.)

 

5) Failing on steps 3 and 4 above, grab your self a pencil and some graph paper. Plot your waypoints at the X,Y locations and write the Z (elev) next to each one. Now, draw lines connecting the waypoints to form triangles to the closet points. Be sure that no lines cross. Now, using linear interpolation, plot each even contour point along the triangle edges using the scaled distance between the points and the difference in elevation between the same points (hint: Y = MX + . Now, connect all the points that have the same even contour value, using common sense so that no contours cross. Done. This is not a hard as it sounds as this is exactly how most contours maps were made by cartographers for hundreds of years.

 

There is variation on this that is half way between the automatic and manual DTM techniques. This will only work if the island is barren. It is an old survey technique called chasing contours. Using the GPS, move to some point at an even elevation you want. Now carefully walk and record a track such that your path follows the same elevation. This will take some practice, but, when you get back to where you started, your track log is the exact contour. Then move to the next contour interval you want, keep and eye on the elevation and follow that track around the island. When done, download your all your tracks (including the shore line) into your map program..... Instant contour plan.

[+RiverCacher]

"Mapsend Topo shows some contour lines on some of the islands in the James River. Which island is it? Do you have any coordinates?"

 

Don't have the coords handy (they're at work).

 

Look for Lynchburg in South Central Virginia. The island is about 10 miles west (upriver) of lynchburg, just upstream from where Salt Creek enters the James on the North Bank. The island is distinctive as it appears that the river made quite an effort to go around its North side...unlike most islands that are simply overgrown sandbars.

 

Photos and maps at www.blackwatercreek.org/baldeagleisland

 

While the island is known as Bald Eagle Island, I have yet to find a modern map that gives it a name at all.

 

Scott

 

Lynchburg, Virginia

[Forester II]

The easiest way is to get a stereo pair of air photos. They are availalble though various sources (state, county, USGS and private firms). Obtain use of a stereo scope or purchase an inexpensive one. Once you get the photos lined up you will see the island in 3 dimensions. It is easy to trace on a mylar overlay the boundary of the island. You can get coordinates with you GPS to tie it down, and use step five from above to draw your contour lines if you think you need them.

[+Rebel]

Coords: N37 30.460, W79 13.807

 

 

Delorme Topo USA 4.0

 

[Kerry.]

quote:

---

Originally posted by MrGigabyte:

 

.... 2) On the island, record waypoints at the significant "break points". These are all the high points, low points, ditches, ponds, outcrops, top of banks, bottom of banks, etc. You cannot have enough waypoints. These point are used to build a "TIN" model (Triangulated Irregular Network). The plane formed be 3 proximity waypoints should be perpendicular to the contours, if you took your waypoints in the right spots. Download your waypoints and track logs. Convert them into a delimited text file of X, Y, Z coordinates.

---

 

For an SPS handheld that's a big ask and quite frankly a waste of time.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+RiverCacher]

"Most river islands are pretty level."

 

You have the correct island there. Yes, most islands are pretty level, as they are simply sand or gravel bars that are held in place by vegetation. This one is different, and the topo maps don't show it being different, which is why we're hoping to make a new map.

 

Thanks for all your help, guys!

 

Scott

 

Lynchburg, Virginia

[marty621]

A 2 acre island, if it were square would be 295 feet square, a very small island. The best results would, and probably the quickest, would be to do a traditional topographic survey of the island. The results would be far more accurate than handheld GPS units presently in use could produce. Unless its heavily wooded or otherwise impenetrable, the field work would take no more than a half day. GPS doesn't make all jobs easy.

[+cachew nut]

The Mapsend map wasn't much different from the Delorme map above, sorry

[Kerry.]

quote:

---

Originally posted by marty621:

A 2 acre island, if it were square would be 295 feet square, a very small island. The best results would, and probably the quickest, would be to do a traditional topographic survey of the island. The results would be far more accurate than handheld GPS units presently in use could produce. Unless its heavily wooded or otherwise impenetrable, the field work would take no more than a half day. GPS doesn't make all jobs easy.

---

 

That's probably about the best suggestion so far as quite frankly a handheld GPS simply wouldn't even come close to making any useful as far as contours are concerned. The handheld SPS Z would simply be totally too eratic and frankly useless.

 

SPS Handheld, total waste of time.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+Rebel]

quote:

---

A 2 acre island, if it were square would be 295 feet square

---

 

That comes to 87,120 square feet. Not too small, lots of room.

 

You might try going to your local community airport and see if a pilot will give you a one hour ride over the island (cost probably less than $100). Take plenty of pictures and video. Then, take the results to the high school or community college art department and see if they can do a composite in three D for you as a class project. I've done these kinds of things before and always get plenty of cooperation.

 

Just an idea.

 

[JDMC]

You could use USAPhotoMaps (at http://jdmcox.com/), which downloads aerial photos or topo maps from the Terraserver Web site, and also can show elevations and contour lines if you download elevation data from a Web site (see Advanced Help).