Garmin GPSMAP 421

Screen and Display Quality

Garmin has equipped the GPSMAP 421 with a 4 inch screen.   The pixels on the display are 240 x 320, which provides a nice picture.  Images are clear and the display is plenty bright enough to give high quality pictures in bright conditions.   A unique feature of the GPSMAP 421 is that unit is waterproof, and performs well under harsh conditions.  Split screen GPS and Sonar works well on the display and allows you to select multiple GPS views to use along with the sonar images.


The Garmin GPS has a built in GPS antenna.  The base map pack that comes with the unit uses satellite imagery, instead of maps.  The GPSMAP has one SD card slot to accept map packs.   Compatible maps are BlueChart g2, Garmin Lakes Vision, Upper Midwest Fishing Guide, Lake Murray and Lake Winnipesakee, Inland Lakes Boundary Waters, U.S. Inland Lakes, Canada Inland Lakes, Homeport, Inland Waterways, and Many Topo Maps.  The unit supports NMEA 2,000, which is nice since the fishfinder can connect to NMEA devices in addition to the Garmin accessories.   The GPS mode on the display uses the Map packs to create 3-d Maps when using BlueChart g2 map packs.  There are two modes, one which allows for an above water 3-d topo map, the other mode allows for underwater 3-d topo maps.  Garmin GPS systems are a favorite among automotive GPS, and the team at Garmin has done a nice job of adapting their specialty to marine use.


The sonar on the Garmin GPSMAP 421 is a dual frequency transducer.  The transducer operates at a frequency of 50 kHz and 200 kHz, which provides a clear image of both shallow and deep water.  The maximum depth that the transducer works is 1,500 ft.  The beam width is adjustable, ranging from 40 to 10 degrees and will mount on a transom that is from 0 to 70 degrees.  By using UltraScroll readings can be given at higher speeds while on plane.   The See-thru technology helps anglers distinguish fish from structure, which helps when fishing heavy structure.


Garmin has lived up their billing as one of the best GPS manufacturers.  The functionality of the GPS features are the highlight of the unit.  The sonar works well, the 200 kHz and 50 kHz signals provide high quality pictures.  One of the drawbacks is that Garmin doesn’t support side scan imaging, which are available on some competitor units.  However the price of the side view imaging on those units put the price point much higher than the GPSMAP 421s.  When compared to other products in the same price range the GPSMAP 421s offer the addition of a GPS where many other fishfinders in the same price range would only be available with sonar.  The GPSMAP 421 offers a great sonar and GPS package at a price most fisherman can afford.