North True Grid Magnetic Astronomical?
The Therion book states .The declination is positive when the magnetic north is east of true north. and north <true/grid> . specify default orientation of the map. By default, true (astronomical) north is used. It is ignored when used with local coordinate system. I'm confused. Presumably almost all cave surveys start out aligned to magnetic north because most of us use compasses that adopt the local magnetic north as their datum. Then, often, we either align the survey to true (astronomical) north manually by specifying a declination, Or align the survey to grid north by specifying a co-ordinate system (that calculates declination automatically) The grid north is usually not the same as the true (astronomical) north, but for the purposes of cave maps and Therion I would assume it is usually considered to be so. (Not strictly correct, but I imagine the error would not be detectable on any printed cave map in most co-ordinate systems) So what is intended by Therion? Presumably some distinction between magnetic and grid north? I have experimented and found that 'north grid' 'north true' appear to produce identical outputs regardless of whether a 'co-ordinate system' or a 'declination' or a 'rotation' is specified. Is there a mistake or am I not understanding something? Regards Bruce
On Sun, Jul 27, 2008 at 2:00 AM, Bruce Mutton <bruce.mutton@paradise.net.nz> wrote:
Presumably almost all cave surveys start out aligned to magnetic north because most of us use compasses that adopt the local magnetic north as their datum.
Then, often, we either align the survey to true (astronomical) north manually by specifying a declination,
Exactly
Or align the survey to grid north by specifying a co-ordinate system (that calculates declination automatically)
No. You usually align the survey to grid north if you are using theodolite to measure angles and your measurements are based on points with known coordinates in the coordinate system used ("grid"). The reason why the grid north is not parallel with true north is the projection of the spherical Earth surface into plane. There is usually one meridian where grid north is exactly the true north, but if you move east or west from this meridian there will be increasing difference between both (called meridian convergence). The compass readings are converted to grid north by applying both the declination (local) and the meriadian convergence (local) corrections.
The grid north is usually not the same as the true (astronomical) north, but for the purposes of cave maps and Therion I would assume it is usually considered to be so. (Not strictly correct, but I imagine the error would not be detectable on any printed cave map in most co-ordinate systems)
The difference is up to 3 degrees for UTM projection or up to 10 degrees for Czech Krovak projection.
So what is intended by Therion? Presumably some distinction between magnetic and grid north?
As described above, the inclusion of meridian convergence effect.
I have experimented and found that 'north grid' 'north true' appear to produce identical outputs regardless of whether a 'co-ordinate system' or a 'declination' or a 'rotation' is specified.
It could be the case if you accidentally happened to live near the central meridian of your coordinate system (grid); e.g. longitudes 3, 9, 15 etc. degrees east for UTM. Martin
Thanks for the comprehensive explanation Martin. So just to clarify if I understand correctly; If we want to plot our maps with respect to magnetic north (to be compatible with 'old' uncorrected surveys for example) then we do not specify declination and we do not specify any co-ordinate system. If we specify a grid to be plotted it will be aligned with magnetic north. Also if we do specify a co-ordinate system and; a) adopt Grid North, the north arrow would plot parallel to the (NS) grids, or, b) adopt True North, the north arrow would NOT (necessarily) be parallel to any of the grids. Thanks Bruce
On Mon, Jul 28, 2008 at 12:20 PM, Bruce Mutton <bruce.mutton@paradise.net.nz> wrote:
So just to clarify if I understand correctly;
If we want to plot our maps with respect to magnetic north (to be compatible with 'old' uncorrected surveys for example) then we do not specify declination and we do not specify any co-ordinate system. If we specify a grid to be plotted it will be aligned with magnetic north.
It would be true if there were no changes in declination. As the magnetic declination changes over time, you should perhaps specify a difference of declinations for old and new data to have them both aligned.
Also if we do specify a co-ordinate system and; a) adopt Grid North, the north arrow would plot parallel to the (NS) grids,
Here the grid is exactly parallel to the page margins and the north arrow is (usally slightly) rotated.
or, b) adopt True North, the north arrow would NOT (necessarily) be parallel to any of the grids.
Here the north arrow is parallel with left/right paper margin and the grid is rotated. Martin
Martin Hmm. So to further clarify the discussion appended below; I appreciate the declination varies with time, but for most potentially active projects in NZ the difference is less than 2 degrees, and if we are dealing with magnetic bearings (historic maps) we accept that any degree of accuracy will be fairly nominal. Does the North Arrow plotted by Therion always point to true (astronomical) north regardless of any of the other settings? Your response to a) & b) below suggests it is so. This would be different to the usual practice in NZ as I understand it, where we plot an arrow to grid north with, often, a secondary arrow to magnetic north as it was at the time of surveying. (Yes, if Therion were to do this it would need to plot an average or a range of magnetic norths) To adopt a co-ordinate grid and then point to true (astronomical) north seems counter intuitive to me, but I accept that there are many different ways of doing things. Therions current behaviour is probably fine, I just need to know what it is! Perhaps the Therion book entry on North true/grid could be updated to clarify the direction the north arrow points. Thanks again Bruce -----Original Message----- From: therion-bounces@speleo.sk [mailto:therion-bounces@speleo.sk] On Behalf Of Martin Budaj Sent: Monday, 28 July 2008 11:39 p.m. To: List for Therion users Subject: Re: [Therion] North True Grid Magnetic Astronomical? On Mon, Jul 28, 2008 at 12:20 PM, Bruce Mutton <bruce.mutton@paradise.net.nz> wrote:
So just to clarify if I understand correctly;
If we want to plot our maps with respect to magnetic north (to be compatible with 'old' uncorrected surveys for example) then we do not specify declination and we do not specify any co-ordinate system. If we specify a grid to be plotted it will be aligned with magnetic north.
It would be true if there were no changes in declination. As the magnetic declination changes over time, you should perhaps specify a difference of declinations for old and new data to have them both aligned.
Also if we do specify a co-ordinate system and; a) adopt Grid North, the north arrow would plot parallel to the (NS) grids,
Here the grid is exactly parallel to the page margins and the north arrow is (usally slightly) rotated.
or, b) adopt True North, the north arrow would NOT (necessarily) be parallel to any of the grids.
Here the north arrow is parallel with left/right paper margin and the grid is rotated. Martin _______________________________________________ Therion mailing list Therion@speleo.sk http://mailman.speleo.sk/mailman/listinfo/therion
On Mon, Jul 28, 2008 at 10:39 PM, Bruce Mutton <bruce.mutton@paradise.net.nz> wrote:
Does the North Arrow plotted by Therion always point to true (astronomical) north regardless of any of the other settings? Your response to a) & b) below suggests it is so. This would be different to the usual practice in
Yes, exactly.
NZ as I understand it, where we plot an arrow to grid north with, often, a secondary arrow to magnetic north as it was at the time of surveying. (Yes, if Therion were to do this it would need to plot an average or a range of magnetic norths) To adopt a co-ordinate grid and then point to true (astronomical) north seems counter intuitive to me, but I accept that there are many different ways of doing things.
The grid north is always indicated by crosses/grid lines, so we use the north arrow to indicate astronomical north exclusively. I think the true north should be always displayed, because the grid north depends on your choice of geodetic coordinate system. As you mention, the magnetic north is ambiguous so we do not display it at all.
Therions current behaviour is probably fine, I just need to know what it is! Perhaps the Therion book entry on North true/grid could be updated to clarify the direction the north arrow points.
OK, it will be more verbose. Martin
Hello all, I've got one question. I would like to draw "line wall -subtype bedrock" say 1mm thick and "line wall -subtype sand" 0.5mm thick. How can this be done? I tried to use command "size". But it seems this command does not work with linetype wall. Greetings, Paul
I am afraid, you need to redefine your symbol-set using metapost code. Just add following lines to your layout: code metapost def l_wall_bedrock(expr P) = T:=identity; pickup pencircle scaled 1mm; thdraw P; enddef; def l_wall_sand(expr P) = T:=identity; pickup pencircle scaled 0.5mm; thdraw P; enddef; endcode HTH, S. Paul Karoshi wrote:
Hello all,
I've got one question. I would like to draw "line wall -subtype bedrock" say 1mm thick and "line wall -subtype sand" 0.5mm thick.
How can this be done?
I tried to use command "size". But it seems this command does not work with linetype wall.
Greetings, Paul
_______________________________________________ Therion mailing list Therion@speleo.sk http://mailman.speleo.sk/mailman/listinfo/therion
Martin Budaj wrote:
The grid north is usually not the same as the true (astronomical) north, but for the purposes of cave maps and Therion I would assume it is usually considered to be so. (Not strictly correct, but I imagine the error would not be detectable on any printed cave map in most co-ordinate systems)
The difference is up to 3 degrees for UTM projection or up to 10 degrees for Czech Krovak projection.
I would add to Martin explanation that while the meridian convergence is fixed (for a given location), the declination varies significantly with time. So if you are managing a cave survey project for say, 10 or 20 years... then the difference in declination will be significant (can be several degrees) Eric.
Eric wrote
I would add to Martin explanation that while the meridian convergence is fixed (for a given location), the declination varies significantly with time. So if you are managing a cave survey project for say, 10 or 20 years... then the difference in declination will be significant (can be several degrees)
Eric.
Eric I guess we are lucky in NZ, the declination varies by about a degree a decade, and we've only been producing maps for about four decades. The overall accuracy today of most of our surveys is probably no better than this (although we would kid ourselves otherwise). Bruce
Hi Bruce, Well, I don't agree with you... There is a difference between the accuracy of your measures (that correspond to probabilistic errors within a given estimation), and a systematic error, as the one you would introduce if you don't take into account the variation of declination between two surveys that would have been done 10 years apart... So this error is not included in your measure uncertainty but it adds to it. Moreover, in most survey programs, it is more or less easy to take into account the declination, or even compute it from the date... It is even easy to add this information a posteriori, if you get old survey data, as soon as you have the date. Eric. Bruce Mutton wrote:
Eric wrote
I would add to Martin explanation that while the meridian convergence is fixed (for a given location), the declination varies significantly with time. So if you are managing a cave survey project for say, 10 or 20 years... then the difference in declination will be significant (can be several degrees)
Eric.
Eric I guess we are lucky in NZ, the declination varies by about a degree a decade, and we've only been producing maps for about four decades. The overall accuracy today of most of our surveys is probably no better than this (although we would kid ourselves otherwise). Bruce
_______________________________________________ Therion mailing list Therion@speleo.sk http://mailman.speleo.sk/mailman/listinfo/therion
Eric You are strictly correct of course. However there are cases where all we have is a paper map, no computer, usually no date (well we do have a date within a decade or so), certainly no survey data, and where survey practices result in gross errors that swamp any probabilistic or systematic ones. It's good to air the rigorous approach as well as the casual. (Usually I take the rigorous side) Thanks Bruce -----Original Message----- From: therion-bounces@speleo.sk [mailto:therion-bounces@speleo.sk] On Behalf Of Eric Madelaine Sent: Wednesday, 30 July 2008 12:09 a.m. To: List for Therion users Subject: Re: [Therion] North True Grid Magnetic Astronomical? Hi Bruce, Well, I don't agree with you... There is a difference between the accuracy of your measures (that correspond to probabilistic errors within a given estimation), and a systematic error, as the one you would introduce if you don't take into account the variation of declination between two surveys that would have been done 10 years apart... So this error is not included in your measure uncertainty but it adds to it. Moreover, in most survey programs, it is more or less easy to take into account the declination, or even compute it from the date... It is even easy to add this information a posteriori, if you get old survey data, as soon as you have the date. Eric. Bruce Mutton wrote:
Eric wrote
I would add to Martin explanation that while the meridian convergence is fixed (for a given location), the declination varies significantly with time. So if you are managing a cave survey project for say, 10 or 20 years... then the difference in declination will be significant (can be several degrees)
Eric.
Eric I guess we are lucky in NZ, the declination varies by about a degree a decade, and we've only been producing maps for about four decades. The overall accuracy today of most of our surveys is probably no better than this (although we would kid ourselves otherwise). Bruce
On 29.7.2008, at 22:48, Bruce Mutton wrote:
However there are cases where all we have is a paper map, no computer, usually no date (well we do have a date within a decade or so), certainly no survey data, and where survey practices result in gross errors that swamp any probabilistic or systematic ones.
Hi, Bruce, anyway you may calibrate such paper map (in the same way as the sections), add coordinates to the entrance, draw what you have and sometimes in the future somebody will resurvey those cave and correct your data. I'm sure this is the main advantage of "therion method". You may collect all accessible data and correct them part after part. Regards Martin
On 29.7.2008, at 22:48, Bruce Mutton wrote:
However there are cases where all we have is a paper map, no computer, usually no date (well we do have a date within a decade or so), certainly no survey data, and where survey practices result in gross errors that swamp any probabilistic or systematic ones.
.Hi, Bruce, .anyway you may calibrate such paper map (in the same way as the .sections), add coordinates to the entrance, draw what you have and .sometimes in the future somebody will resurvey those cave and correct .your data. . .I'm sure this is the main advantage of "therion method". You may .collect all accessible data and correct them part after part.. . .Regards .Martin Yes Martin, I have started out on this course and this aspect of Therion is working very easily. Thanks heaps. Bruce
participants (6)
-
Bruce Mutton -
Eric Madelaine -
Martin Budaj -
Martin Sluka -
Paul Karoshi -
Stacho Mudrak