Baitboat_Dude's Website

You can use baitboat sonar and GPS to make underwater contour fishing maps

When the baitboat returns to the shore you instruct the PDA software to calculate a map from the file of data it has made.  It then calculates and draws the depth contours on the map.  Seconds later you have a fishing map, with accurate depths all around you. If you go out in a boat to see a feature shown on your lake map, the PDA GPS navigation will give you accurate instructions how far, what direction over the featureless water surface, until you are right over, (or right in front of - doesn't scare the fish ! ) the underwater feature, dropoff, cliff, trench, whatever.

Not a prediction of the future... it's a reality.   take a look here ... How to Get Accurate Contour Maps of the Lake Bottom Using DrDepth

I use this system from small boats.  But some places do not allow boating, so I have designed a baitboat to carry my lake mapping equipment over the lake making the fishing maps automatically for me while I fish from the shore.   More details to follow soon.

Meanwhile .. .if you are under any doubts as to the value of accurate fishing maps take a look here at the results Bruce Samson has had with Lawrence sonar-GPS using contour maps for winning big US fishing competitions.

The software you need to make your own GPS fishing maps of your local lakes and rivers.
Not many folks want to get into making their own maps, but I've been doing it for a long time now. Just getting myself off from paper and onto sonar-GPS digital took a bit of spare time study.

The Lowrance .lcm file format (also sometimes as at5 format) has also been hacked and "reverse-engineered".

LcmMapedit is the freeware program for making Lowrance map files. LcmMapedit is a modified version of GPSMapedit but with the lcm Lowrance file type authoring added. It produces the lcm file ready for uploading, so the Garmin requirement of final conversion into an img file is not necessary. It is downloadable at the mapcreate forum for lowrance map makers http://tech.groups.yahoo.com/group/mapcreate_format/

Make your own GPS fishing maps automatically with a PDA handheld and sonar unit
Drdepth software from www.drdepth.se does a nice job of collecting the depths and gps locations directly from the sonar/GPS unit. A Lowrance LMS or LCX series sonar has the ability to record the depth/GPS info on a SD card, so they can be copied to a PC later. Then Drdepth puts them into a database, and will produce the depth contours if desired.
3Dfield http://field.hypermart.net/ also freeware, does a nicer job, but while technically more capable, is less user friendly.
The PDA and sonar unit can work a few different ways:
PDA with GPS inside + Sonar (only)
PDA (cheaper - no GPS) + Sonar-GPS Combo unit
PDA with GPS inside + Sonar-GPS Combo unit .. you choose which gives the most precise GPS location data and select that for the map.

Making fishing maps for displaying on your waterproof GPS-Sonar Combo unit - without a PDA.
If anyone wants to try doing their own GPS mapmaking using GPSMapedit, here are two links to video tutorials showing the process for Garmin.
video tutorial 1 http://s173.photobucket.com/albums/w59/redhawkdown/?action=view&current=GoogletoCustomGarminMap.flv
video tutorial 2 http://s173.photobucket.com/albums/w59/redhawkdown/?action=view&current=GoogletoCustomGarminMap2.flv
The Lowrance process is virtually identical, but without the final part, as LcmMapedit gives the finished map.  Lowrance map makers can download the Lowrance version of GPSMapedit from the files section at the Lowrance Map Makers Forum:  http://tech.groups.yahoo.com/group/mapcreate_format/ . 
Don't let the apparent complexity of this put you off - it is quite easy when you have tried it. And the results while fishing are well worth the trouble. Just look at the videos, then download the free software, then have a go. The online tecchie guys are helpful if it doesn't work out properly the first time.
 Garmin are in the process of changing their map file format, so I think that Lowrance and Eagle have the advantage at the moment for the private mapper. The higher Humminbird Matrix, 700 and 900 series units also export LMEA0183 format depth data via cable so these can also do the job when coupled with a PDA and DrDepth.
Right now if you want to display your map on the sonar unit itself, you had better get a Lowrance/Eagle, since the 'Bird will need the PDA to display the map, as described in the paragraph above. And on a rainy day this is dangerous to the PDA.
An extra stage added in this video is tracing an outline from Google earth at the start.  Something similar would be done if you want to trace an outline of the lakeshore from an aerial photo.  I found these videos very useful in getting an understanding on the process of making maps of the lakes I fish.