Seeking GMRS Repeater Coverage Map with Topographic Details

[WSFQ498]

Just got my GMRS license.  I'm looking for map that will show repeater coverage according to actual topography. Where can I find it?

[WRUE951]

10 minutes ago, WSFQ498 said:

Just got my GMRS license.  I'm looking for map that will show repeater coverage according to actual topography. Where can I find it?

There are several on-line tools. Try Splat..  You will need coordinates.  You can get that off the KML maps if your are a paid member..   Or if you know how to read nd decode the site HTML data you can pick site coordinates from there...   

[Socalgmrs]

Keep in mind that all these site really are only accurate to a point and the. You’ll have to get out in the field and out boots on the ground.  It also has zero bearing on if you can hit the repeater from any of those areas.  

[808Beachbum]

Probably a more practical idea would be to actually go out and test your rig, perhaps with someone on a second radio, stationary at your home location, then drop pins in googleearth so you can clearly determine line of sight distances and elevations, as well as obstructions that may affect your actual range. This has the added benefit of providing actual experience communicating and learning that sometimes you only need to move 50 feet in one direction or another to have acceptable comms.

[WRVR303]

I'm not aware of any sort of comprehensive GMRS coverage map / data base, probably because repeaters are owned by private individuals who all operate independently of each other. They may have their own maps, but they are not all conglomerated. So, you have to figure out where the repeaters of interest to you are, how high the antennas are off the ground, and then make your own maps. Note that very few, if any, of the repeaters shown on the MyGMRS maps are in the actual location shown, so any plots based on that info will be misleading. 

And, any calculated coverage map is still "just a (computerized) guess".

[WRYZ926]

Coverage maps like you see on MyGMRS maps and repeater book are just good estimates. The only real way to determine coverage area is to get out and test. Our MGRS repeater antenna is on a local radio station tower at 400 feet above ground. We have a 35 mile radios coverage area. But local terrain and forested areas can and will make a difference on if you can actually get into a repeater. Several of us drove around and tested our repeater's coverage area before opening it up to public use.

Our 70cm repeater has about the same overage ares even though its antennas are at 900 feet above ground. Our 2m repeater antennas are also at 900 feet above ground and we get close to a 80 mile radios of coverage. Just to show the difference between VHF and UHF. The 70cm and GMRS repeaters are putting out 30-35 watts after the duplexers while the 2m is putting out around 45 watts after the duplexer.

[WRKC935]

Accurate coverage mapping requires a number of different pieces of information to run the calculations against to generate a map.

Some of those are power from the transmitter, duplexer loss, antenna cable loss, antenna gain, antenna height at the center of the radiation pattern, antenna pattern and down tilt.

What is calculated is path loss based on the transmit and receive frequencies, losses due to vegetation, buildings and ground cover.  This is selected in the coverage mapping software during the configuration and data input.  You will also need to supply the subscriber antenna gain, transmit power and antenna height so that the talk in can be calculated.  A base radio will work farther than an mobile with the same power output because of antenna height, a mobile will work better than a portable due to antenna gain and power output from the portable being typically 10dB less than the mobile. 

Again, there is software out there to run the numbers.  But without the REAL numbers, it's garbage in and garbage out.