Offering To Do Another Faq

[itchytweed]

I realize that this may have been covered before elsewhere and I may not have seen it. If agreed upon, I can write another FAQ about using the laws of sines and cosines to get all angles for a station and its associated marks. Here again, I would use my own personal devil, WAUWA, as the example. I have not seen a station that uses a right angle in my area to set the reference and azimuth marks.

 

5 degrees out and a windchill at -10 F is not conducive to BM hunting and taping off positions so I might as well type and share some knowledge.

[+Black Dog Trackers]

itchytweed -

 

That sounds like it might be usable, but for lack of the necessary equopment, most of us can only make use of compass bearings, not turning angles.

 

I suggest a spreadsheet or something that would accept manual input of:

1. the local declination

2. the distance and true-North bearing from the main station to each ref. mark and AZ mark

and would output:

a. the coordinates of each ref. mark and the AZ mark

b. the distance and magnetic (compass) bearing from the main station to each ref. mark and AZ mark

c. the distance and magnetic bearing to the main station and other ref. marks from each ref. mark

d. #a, #b, and #c again in case the PID is one of those that uses the sometimes-used surveyor system of 180 degrees = true-North.

 

Such a spreadsheet would be good for those second trips to particularly frustrating sets of disks.

The declination isn't important in all areas of the U.S. for handheld compass precision, but for those other areas, quite valuable, and calculating the results of declination is a pain in the field.

[itchytweed]

BDT -

 

You hit the process on the button. A spreadsheet would be easy to do and can be put into a handheld, laptop or desktop computer.

 

I was going to make it so that you plug in the angles off the datasheet and the output is angles from True North. I don't have the equipment either for turning angles but converting them to angles from True North is the best way to go about this.

 

I will not deal with declination as this should be compensated for by calculating from True North.

 

Now, once the spreadsheet is done, how to pass it on?

[+Black Dog Trackers]

I will not deal with declination as this should be compensated for by calculating from True North.

I don't know why you wouldn't want to add this feature in case someone wanted it. Oddly enough, compensating for declination can be a bit confusing (do I add or subtract?) and I'd rather a spreadsheet did this for me 100% so that all I have to do is use my compass and point it in the appropriate direction. I use a Suunto that measures to ½ degree but doesn't have a declination adjustment in it.

 

As for a destination for your spreadsheet, there are 2 choices:

1. get one of us that has a web page for benchmarks to put it in

2. make a web page of your own to put it in (I used a free site)

[+ddnutzy]

Here's a copy of the declinations in the U.S. If your east of the zero or agonic line you subtract the declination from the compass bearing if your going from magnetic to true and you add the declination if going from true to magnetic. If your to the west of agonic you add from magnetic to true and subtract from true to magnetic.

 

Dave

[+Black Dog Trackers]

Having just attended the benchmark hunting get together, I find that my GPS is an antique in that it doesn't have an internal compass that is automatically oriented to true-North. So perhaps there are many people that don't need to worry about declinations like I do.

 

ddnutzy - thanks for posting the map. I just like the calculation done for me even though it's very simple, it is one less thing to remember to calculate on site. A printout where it is already done would be nice. If your compass already compensates for true-North, the declination cell in the spreadsheet could remain zero.

[+ddnutzy]

Having just attended the benchmark hunting get together, I find that my GPS is an antique in that it doesn't have an internal compass that is automatically oriented to true-North.  So perhaps there are many people that don't need to worry about declinations like I do.

 

ddnutzy - thanks for posting the map.  I just like the calculation done for me even though it's very simple, it is one less thing to remember to calculate on site.  A printout where it is already done would be nice.  If your compass already compensates for true-North, the declination cell in the spreadsheet could remain zero.

Bdt

I have one compass that I always use with the declination set for the area I'm in. I trust the compass more than I trust the electronic compass on my gps 60cs. That thing will bounce around but the compass is always right.

If your looking for a bm that is buried under a hundred years of mulch and dirt and you have a reference from the corner of a wall that is 50 meters at 160 degrees and your declination is 15 degrees like it is in most of Ma. you'd better figure the declination in if you want to find the station. Quite a few of my finds have been found with information from an azimuth from a old stone fence or a big boulder near a 18 inch oak tree. The declination is quite important in situations like that.

 

Dave

[Bill93]

How recent is that map of declinations? I thought I was a lot closer to the current agonic that it indicates.

[+Black Dog Trackers]

Here is a 2000 map.

[+Black Dog Trackers]

This page has 3 sizes of Conterminous 2004 USA maps.

[itchytweed]

NMEA sentence $GPRMC has magnetic declination included in it. Now the question is - Is the value from a lookup table that will have to be constantly updated or is it something that is calculated from the satellites and their data streams?

[+Black Dog Trackers]

itchytweed Posted on Dec 19 2005, 04:27 AM

NMEA sentence $GPRMC

I don't know what that means but I was just meaning that a person using a spreadsheet you make could enter a declination manually if they wanted to get the spreadsheet to make use of it.

[+Klemmer]

Itchy is refering to the National Marine Electronics Association (NMEA) Specification 0183. Commonly referred to as NMEA 183 or sometimes just "NMEA". It has somehow become the de-facto protocol standard for RS-232 or USB Serial Data output and input for GPS receivers (including handheld consumer units). My company even uses it on some military products, and for time syschronizing other products, like digital video / data recorders. There are various versions of the "NMEA" spec, and every GPSr I have seen / used in the last 10 years or so (12+) can be set to use one or several NMEA versions. Sometimes, there will also be available a particular company's protocol as well (i.e. Garmin).

 

Now, exactly WHERE a particualr GPSr get the declination information that is available on most (recent) NMEA protocols, is a question for the manufacturer. I SPECULATE (just my guess) that since the GPSr knows where it is (assuming recent satellite lock), it would b easy enough to have a lookup table built-in. The data would become SLIGHTLY dated (maybe a degree or so) in 3 - 5 years, as the earth's magnetic pole drifts moves. MAYBE new firmware would update it? Certainly depends on the manufacturer. For our typical purposes (to provide a magnetic bearing to a cache), probably plenty accurate enough for the life of the GPSr.

 

Interestingly, Delorme provided an updated declination file a year or so ago for it's mapping product that I use (3D TopoQuads), and I assume for other mapping packages also. I certainly didn't notice any difference, but appreciated it anyway.

Edited December 19, 2005 by Klemmer & TeddyBearMama

[Gnikhog]

Personally I have been using a spreadsheet for horizontal control stations with reference marks since returning from recovering my first mark. My spreadsheet, which has evolved a bit over time, includes only data that are important to me and are not already available either from the NGS datasheet or by using my GPSr proficiently. My spreadsheet simply calculates the azimuth and distance from RM to RM based on the NGS boxscore information and generates a scaled plot of the station and the RMs.

 

I used simple trig functions as the formulas like those used in FORWARD and INVERSE offer precision and accuracy beyond that which I can measure angularly or in a straight line (or need). Besides, I didn't need lat/lon data for the RMs. In other words, my spreadsheet offers data with sufficient accuracy for locating RMs that are typically less than 50 meters from the station.

 

Although I believe that I am probably more proficient with map and compass than most, I know that nothing lies like a magnetic compass especially when the user ignores the environment. (A possible exception is an overdriven FET probe but that is a discussion for another time and place.) While I too preset the declination on my compass, I consider it to be only a relative direction indicator. While some may believe that their compass always points to magnetic north, I know mine doesn't. Because of this, I prefer that my azimuths remain in true north format.

 

I am willing to share my spreadsheet if there is an interest. It isn't elegant, doesn't come with instructions or support, but it has served my needs very well. Of course, it is not immediately evident to me that the message board is set up to allow files to be attached but I dont have a reason to look hard yet.

 

/John

[itchytweed]

I ditched the sines/cosines process in favor of polar/rectangular conversions. The end result is the same but having the X/Y values is good for tape runs, especially when defining a search area for trying to find an elusive monument. I currently have it set up for three RM's/Azimuth but the spreadsheet can be enlarged to fit any amount of references.

 

An added benefit is that the math is far, far easier.

 

Now to build a website to store it on. Happy, Happy, Joy, Joy.

[mloser]

Itchy,

If you want a place to put it I can store it on my brother's server without a problem. He currently hosts the NGS benchmark mini-page to allow easy PID lookup from WAP devices and I am sure would store your sheet for download.

 

For those who want the PID page, go to NGS PID mini-page.