Using NH State Plane on Garmin GPS

[psmart]

I've replaced my old Magelan ColorTrak with a new Garmin 60CSx, partly because of it's popularity on VFTT. The 60CSx is an improvement in all regards, except that the user coordinate system cannot be fully configured for NH State Plane Feet.

The first hurdle is that the Garmin does not allow the Latitude of Origin to be specified. However, this can be overcome by incorporating this into the False Northing, as described here: http://gpsinformation.net/main/usr-grd1.htm

But the Garmin also lacks a final units conversion factor to convert meters to feet. So I have to use a calculator to do the conversion by hand.

Has anyone else dealt with this? Are there other work-arounds that I may be missing?

BTW: I'm using NH SPF NAD27 because that's the way the USFS mapping continues to be done. I'm aware of sites that can convert (reproject) individual coordinates, but the ideal solution would be to read SPF directly on the Garmin, like my old Magellan.

 

[SAR-EMT40]

I've replaced my old Magelan ColorTrak with a new Garmin 60CSx, partly because of it's popularity on VFTT. The 60CSx is an improvement in all regards, except that the user coordinate system cannot be fully configured for NH State Plane Feet.

The first hurdle is that the Garmin does not allow the Latitude of Origin to be specified. However, this can be overcome by incorporating this into the False Northing, as described here: http://gpsinformation.net/main/usr-grd1.htm

But the Garmin also lacks a final units conversion factor to convert meters to feet. So I have to use a calculator to do the conversion by hand.

Has anyone else dealt with this? Are there other work-arounds that I may be missing?

BTW: I'm using NH SPF NAD27 because that's the way the USFS mapping continues to be done. I'm aware of sites that can convert (reproject) individual coordinates, but the ideal solution would be to read SPF directly on the Garmin, like my old Magellan.

Holy cow are you specializing.

I never use the state plane datums and like you say I believe all of them are in feet. But like you say the way around it would be to use a customized coordinate system. I know of no specific way to get everything your looking for other than that. I'd be interested in hearing if anyone has a single solution to this other than a customized coordinate system.

Boy this one will have some scratching their heads.

Keith

 

[DougPaul]

I've replaced my old Magelan ColorTrak with a new Garmin 60CSx, partly because of it's popularity on VFTT. The 60CSx is an improvement in all regards, except that the user coordinate system cannot be fully configured for NH State Plane Feet.

We've suckered you in?

The first hurdle is that the Garmin does not allow the Latitude of Origin to be specified. However, this can be overcome by incorporating this into the False Northing, as described here: http://gpsinformation.net/main/usr-grd1.htm

But the Garmin also lacks a final units conversion factor to convert meters to feet. So I have to use a calculator to do the conversion by hand.

Has anyone else dealt with this? Are there other work-arounds that I may be missing?

BTW: I'm using NH SPF NAD27 because that's the way the USFS mapping continues to be done. I'm aware of sites that can convert (reproject) individual coordinates, but the ideal solution would be to read SPF directly on the Garmin, like my old Magellan.

From your link, it appears that you may be aware of the following. If not:

The 60CSx (and many other Garmins) have a user datum and user position format ("User UTM Grid") in which you specify the parameters.

Just tried them on my 60CSx: I selected the user datum/position format and there are a bunch of fields to fill in. Might be more info in the manual.

If this doesn't do it, I suppose that you could request a feature upgrade from Gamin.


There are also some datum conversion websites and programs which might be helpful, if only in general procedures. It might be worth a search or two. BTW, GPSBabel (www.gpsbabel.org) handles datum and format conversions, but doesn't list NH SPF.

Doug

 

[arghman]

Has anyone else dealt with this? Are there other work-arounds that I may be missing?

ick, the multi-datum problem.

I'm not sure how you can configure the Garmin, which I suppose would be ideal if you are manually entering/recording coords in the field, away from a computer.

I use decimal degrees in my GPS, but then convert to/from state plane coords with a program called ExpertGPS. It's not free ($60 for the base software, another $40 = $100 total if you buy the optional GIS pack) but it is a snap to convert/import/export waypoints/shapefiles/etc. I've converted both NH NAD27 and NH WGS84 (or NAD83 or whatever you call the current one) -- which are quite a bit different; I was almost tearing my hair out wondering why I couldn't import the USFS layers into my computer.

Before the ExpertGPS GIS pack, I was using the US Army Corps program "CorpsCon" which also does the conversion, but it was a bit clunky and easy to screw it up if I got the column order wrong. But it is free.

 

[DougPaul]

I use decimal degrees in my GPS, but then convert to/from state plane coords with a program called ExpertGPS. It's not free ($60 for the base software, another $40 = $100 total if you buy the optional GIS pack) but it is a snap to convert/import/export waypoints/shapefiles/etc. I've converted both NH NAD27 and NH WGS84 (or NAD83 or whatever you call the current one) -- which are quite a bit different; I was almost tearing my hair out wondering why I couldn't import the USFS layers into my computer.

Check out GPSBabel http://www.gpsbabel.org. It is very versitile. Open source, free, etc. You can also leagally modifiy it to include NP SPF if you so desire.

Doug

 

[sardog1]

In addition to Corpscon, the feds (in this case, the National Geodetic Survey) list GPPCGP for NAD27 conversions and SPCS83 for NAD83.

I haven't used either, as I'm a Mac kinda guy.

 

[arghman]

Holy cow are you specializing.

I never use the state plane datums and like you say I believe all of them are in feet. But like you say the way around it would be to use a customized coordinate system.

All the GIS systems I'm aware of (which only numbers 5 or 6 so I guess I shouldn't generalize, though this includes WMNF, SPNHF, the state's public GIS database = "GRANIT", NH Natural Heritage, my town) use one of the state plane systems.

<begin rant>
maybe it's considered specialized from the consumer end of things, but this seems like another instance of the GPS companies producing lowest-common denominator products aimed at the most common 95% of the market & it makes it harder to use them for natural resources work. If the camera companies made digital cameras the same way and catered to the most common 95% of the market, there would be no hot shoes, manual focus, external lens/filter threads, etc.
<end rant>

 

[DougPaul]

All the GIS systems I'm aware of (which only numbers 5 or 6 so I guess I shouldn't generalize, though this includes WMNF, SPNHF, the state's public GIS database = "GRANIT", NH Natural Heritage, my town) use one of the state plane systems.

<begin rant>
maybe it's considered specialized from the consumer end of things, but this seems like another instance of the GPS companies producing lowest-common denominator products aimed at the most common 95% of the market & it makes it harder to use them for natural resources work. If the camera companies made digital cameras the same way and catered to the most common 95% of the market, there would be no hot shoes, manual focus, external lens/filter threads, etc.
<end rant>

<counter rant>
Uhh... The 60CSx is hardly a lowest common denominator product.

I just counted the number of datums on my 60CSx: 117. The GPS is a worldwide instrument--I can only guess how many datums are in use worldwide: the number could easily be in the thousands or tens of thousands. I think it is a bit unreasonable to expect the manufacturers to include every local datum. To their credit, Garmin does provide for a user defined datum to allow you to install your own local datum.

Garmin customer support is far more responsive than that of many (most?) other companies. You could ask them about programming your favorite local datum into the GPS. Or you could request that they include it in the next software update or publish sets of parmeters for programming a number of local datums into the user datum on their website.

And local organizations/companies might also do well to choose larger area datums (eg well established standards) for their own products/work.
</counter rant>

Much of the reason for local datums is historical. People needed to survey an area but did not have the ability to tie the local datum into a larger/worldwide datum. And now we have many local datums to work with. In theory all old cordinates could be converted to a modern datum, but that costs money and effort.

BTW, there still is and always be reason for multiple datums. Surverying is now accurate enough to be affected by tectonic movements. Some datums try to be tied to absolute location on the planet and place cordinates will be affected by these movements. More local datums move with the local terrain to minimze the effects of the tectonic movements.

Doug

 

[arghman]

<counter rant>
Uhh... The 60CSx is hardly a lowest common denominator product.

I just counted the number of datums on my 60CSx: 117. The GPS is a worldwide instrument--I can only guess how many datums are in use worldwide: the number could easily be in the thousands or tens of thousands. I think it is a bit unreasonable to expect the manufacturers to include every local datum. To their credit, Garmin does provide for a user defined datum to allow you to install your own local datum.

Garmin customer support is far more responsive than that of many (most?) other companies. You could ask them about programming your favorite local datum into the GPS. Or you could request that they include it in the next software update or publish sets of parmeters for programming a number of local datums into the user datum on their website.

And local organizations/companies might also do well to choose larger area datums (eg well established standards) for their own products/work.
</counter rant>

Fair enough. I'll have to give them my wishlist for firmware upgrades. I hope they're better in that regard than Lowrance (which took a few years to fix a simple display bug to show Estimated Probability of Error, and it was like pulling teeth to get them to even acknowledge it was a bug, rather than an instance of "user error").

psmart: best of luck w/ the datum problem. I hope you are able to find a solution that works for you.

 

[DougPaul]

psmart: best of luck w/ the datum problem. I hope you are able to find a solution that works for you.

I think we can agree on this one...

It might also be worth contacting Garmin's tech support to see if they can help with NH SPF.

Doug

 

[psmart]

As I understand it, the user coordinate system in most Magellan GPS models can handle State Plane Feet, while most Garmin's (at least the recreational units) cannot. The Garmin is also limited to "User UTM", while the Magellan menu also offers Lambert, Stereographic, Oblq Merc, and Polyconic user grids. It does seem an odd marketing decision. It might be that Garmin is trying to preserve their market for "professional" units for surveying and natural resource work, but it seems short sighted. Apparently, the state plane grid is also 4x more accurate than UTM for things like distance calculations, so there is some basis for staying with SP rather than UTM when accuracy is critical.

I will contact Garmin support, as suggested, to see if they offer any assistance. If anyone is interested (and in case I loose my notes!) here are the setups I've been using for NH State Plane:

For Magellan (User Grid):
Projection = TM
Latitude of Origin = 42.5 N
Longitude of Origin = 71.666666 W
Scale Factor = 0.99996666
Units to Meters conversion = 0.30480061
False Easting at origin = 500,000 (feet)
False Norting at origin = 0

For Garmin (User UTM Grid):
Longitude of Origin = 71° 40' W
Scale = 0.9999667
False Easting = 152,400 (meters) = 500,000 feet
False Northing = -4,706,805 (meters)

The last value is the "trick". This has been adjusted to incorporate the Latitude of Origin, which cannot be set on the Garmin. (My original post has a link with the details.) But since there is no "Units to Meters" conversion, the resulting coordinates are State Plane Meters, not Feet as generally required with NAD27. (Doing the final conversion on a calculator produces the correct coordinates in feet!) I guess Garmin figured that no one needed feet anymore. After all, NAD84 State Planes are generally in meters.

My head hurts... But thanks for the feedback!

 

[DougPaul]

Apparently, the state plane grid is also 4x more accurate than UTM for things like distance calculations, so there is some basis for staying with SP rather than UTM when accuracy is critical.

It looks like you may be confusing accuracy with precision. Integral feet are ~3x more precise than integral meters, but since the accuracy of a consumer GPS is more like ~10 meters, the extra precision means little. In the context of a survey grade instrument, the difference could be important. (If you can make measurements in fractions of feet or meters, the units become a non-factor.)

I will contact Garmin support, as suggested, to see if they offer any assistance.

Worth a try--the worst they can tell you is "you can't get there from here".

But since there is no "Units to Meters" conversion, the resulting coordinates are State Plane Meters, not Feet as generally required with NAD27. (Doing the final conversion on a calculator produces the correct coordinates in feet!)

Finishing the conversion externally at least enables you to verify the rest of the conversion.

I guess Garmin figured that no one needed feet anymore. After all, NAD84 State Planes are generally in meters.

Beats me...

My head hurts... But thanks for the feedback!

Take 2 asprin and call Garmin in the morning...


If worst comes to worst you could connect the 60CSx, set to output NMEA, and a PDA. The NMEA (a text mode) would contain the location in WGS-84 (I think) and you could do the conversion automatically on the PDA.

Doug

 

[arghman]

For Magellan (User Grid):
Projection = TM
Latitude of Origin = 42.5 N
Longitude of Origin = 71.666666 W
Scale Factor = 0.99996666
Units to Meters conversion = 0.30480061
False Easting at origin = 500,000 (feet)
False Norting at origin = 0

For Garmin (User UTM Grid):
Longitude of Origin = 71° 40' W
Scale = 0.9999667
False Easting = 152,400 (meters) = 500,000 feet
False Northing = -4,706,805 (meters)

Hmm. I don't fully understand how the custom datum thingy works, but maybe you can get what you need by tweaking the "Scale Factor" from approximately 1.0 to the foot/meters conversion factor. NIST SP811 says 1 US survey foot = 0.3048006 m, therefore 1m = 3.280833 US survey feet. One of those 2 scale factors (+ the appropriate multiplication/division to the false easting/northing) ought to make things work in feet rather than meters. I can't help you with whether to multiply or divide; I have parity problems & get it wrong about 75% of the time.

 

[psmart]

maybe you can get what you need by tweaking the "Scale Factor" from approximately 1.0 to the foot/meters conversion factor.

I tried it both ways and got "invalid position" or something to that extent. It probably produced an out of range value. However, I didn't try all the possible combinations of feet/meters for the other parameters.

 

[psmart]

It looks like you may be confusing accuracy with precision. Integral feet are ~3x more precise than integral meters, but since the accuracy of a consumer GPS is more like ~10 meters, the extra precision means little.

Actually, I was referring to the accuracy of the projection itself, and resulting distance measurements. Apparently distance measurements (nothing to do with GPS or the number of digits) are considerably more accurate when calculated from State Plane coordinates than from UTM. (There is less distortion in the projection.) As you point out, this distinction doesn't really matter for GPS use, but apparently it's one of the reasons the State Plane systems continues to be used instead of UTM or other alternatives.

From this interesting discusion of the State Plane system:

 

[sardog1]

All the GIS systems I'm aware of (which only numbers 5 or 6 so I guess I shouldn't generalize, though this includes WMNF, SPNHF, the state's public GIS database = "GRANIT", NH Natural Heritage, my town) use one of the state plane systems.

NH is a little unique in using feet, at least regionally. Vermont and Mass. both use their state plane coordinate systems, based on NAD83, but their standards use meters, not feet. Maine's standard is UTM, NAD83, Zone 19, meters, not a state plane coordinate system.

It's totally a WAG on my part, but I suspect that the WMNF folks use the NH state plane reference as an accommodation to the locals (e.g., for the WMNF road layer that's published.)

 

[DougPaul]

Actually, I was referring to the accuracy of the projection itself, and resulting distance measurements. Apparently distance measurements (nothing to do with GPS or the number of digits) are considerably more accurate when calculated from State Plane coordinates than from UTM. (There is less distortion in the projection.) As you point out, this distinction doesn't really matter for GPS use, but apparently it's one of the reasons the State Plane systems continues to be used instead of UTM or other alternatives.

From this interesting discusion of the State Plane system:

OK. Just read the reference.

IMO, UTM is a bit of a hack, forcing a section of the surface of a sphere into a flat representation. The state planes just use smaller regions and thus have smaller errors.

I'm a lat-lon boy myself... (due to a nautical navigation background)

Doug

 

[JOD]

coordinate conversion

Boy oh boy! Do you really want to convert the coordinates? I use Corpscon 6.0 for work. It's FREE because we've all already paid for it with our taxes. I've been using Corpscon for about 7 years. It's had a few updates in that time. You can create space delimited files and do multiple waypoints at once.

 

[Jason Berard]

Thanks to all who posted in this thread.

I had to convert some points from Northing/Easting so I could input them into Mapsouce. I wanted to see just generally where I was going to be headed on my next Boundary trip, and this really helped a lot! Thanks again to all. I may be the least tech savvy person I know, and I could follow the directions on Corpscon. Good stuff.