Last week, I visited the USGS site and the Acme mapper site and pulled up their topo. maps of Lake Fork. (It turns out that their maps are identical). I recorded the coordinates of several creek locations (bends and junctions) and several roadbed locations (bridges) in Birch Creek and the old 2946 roadbed area.

I then entered the coordinates as waypoints in my HDS-8.

When I switched to the Lowrance mapping option in the HDS-8 and zoomed in on the waypoints I had entered. The locations matched exactly at the 200 ft. zoom level.

From this I concluded that the Lowrance mapping (which displays creek channel and roadbed information) was likely derived from USGS topo. map data just as my entered waypoints had been.

A few days later I made a trip to Lake Fork and while there, I made it a point to try to find the waypoints I had entered using my HDS-8/LSS-1 Side Screen view.

I found the roadbed points I had entered fairly easily and at the 500 ft. zoom level, they appeared to coincide exactly with the Lowrance map.

Zooming in closer (to the 100 ft. zoom range) told a bit of a different story. When I found the roadbed features using the Side Scan and marked waypoints, I found that the waypoints entered by the Side Scan differed from the topo./Lowrance waypoints by varying amounts with the closest being 27' apart and the worst being 74' apart.

Locating the creek waypoints I had marked turned out to be a problem. I was unable to correlate what I was seeing on the Side Scan with what the maps said should be there.

The GPS Lake Maps discussed in a couple of other threads are said to also be derived from USGS topo. map data. This leads me to the conclusion that I already have map information loaded on my HDS unit that shows creek channel and road bed information (e,g. the Lowrance Mapping) and since it evidently was derived from the same source is most likely to be of the same relative accuracy. Further with the accuracies I noted by locating some of the features with side scan, those maps will not generally get me close enough (e.g., consistently within casting distance) to track the feature (creek or roadbed) using only the map view.

For Bobby Feazel (AKA kestrel63): Is the GPS LAKE MAP information you provide likely to be any more accurate than what I was able to obtain directly from on-line topo. maps? If I do the same experiment with your map, am I likely to see more accurate results?

C.

Most paper maps and map software are derived from USGS maps rather than independent surveying.

Most people, me included, find it easier to pay the $ and then plug & play rather than do the homework you have done.

Great job!!!!!!!!

C.

Great study. I�ll be doing some of the same things when I get my side scan.

GPS Lake Maps are constructed from historical USGS topo maps as are all others that I know of.

"Are they more accurate than others?" Not giving you a smart answer here, but "sometimes they are and sometimes they aren't because any combination of errors at the same location can either cumulate or compensate." And don't forget there is always some position error in any GPS unit that can add to or take away from the combined error.

The reason there are variances in all these maps is because there is human error mixed in from the day the original surveys were made to the day that the new maps were made. The USGS maps were made by cartographers from many aerial photographs and checked by ground surveys. Lots of time, lots of effort by lots of people with lots of opportunities to be off.

In most cases, the biggest advantage to a GPS Lake Map is that one can now have both detailed contours and creek channels shown on their GPS at the same time.

I don't know about your HDS base map but most units that show creeks on their base map don't have detailed contours. In other words when one installs his Navionics card it takes the place of the base map leaving one to either use one or the other or switch between the two.

Some of the older Navionics cards do show very thin purple lines depicting creeks and I find them to be generally as accurate as GPS Lake Maps but they are very hard to see.

For my way of fishing, my first priority is to have detailed contours and my second is to know where the creek channels are. But I want to see both at the same time.

[quote=Bobby Feazel (AKA kestrel63)I don't know about your HDS base map but most units that show creeks on their base map don't have detailed contours. In other words when one installs his Navionics card it takes the place of the base map leaving one to either use one or the other or switch between the two.

Some of the older Navionics cards do show very thin purple lines depicting creeks and I find them to be generally as accurate as GPS Lake Maps but they are very hard to see.

[/quote]

The built-in mapping in my HDS-8 is the Insight version which appears to be identical to the Lowrance basemap that is on my HDS-7 with respect to lake features such as creek channels and roadbeds. You're correct that these maps do not have close contour lines -- just 10' intervals.

My Navionics map for Lake Fork gets little use because so much of the lake doesn't have the closer contours and I've found so many areas where the contours shown are just flat wrong!

C.