You and Loyal would get along famously. You mentioned Wang computers last week. He started using LDP's on a Wang. He taught the nuts and bolts of LDP design to me. I owe him a lot of beer. Loyal's two cents convert to at least a buck fifty in anyone else's currency here at surveyorconnect.
Distortion IS
> ...as distortion DOES.
> This semantic sideshow is only going to further confuse the confused.
Not really. The LDP crowd have already thoroughly muddied the water by not recognizing that the topographic surface is not where the latitudes and longitudes of points are located with respect to NAD83, but on the reference ellipsoid.
The fundamental problem of map projections is a one-to-one mapping of points on that ellipsoid to a developable surface, meaning one that could in theory be rolled out flat onto a cartesian coordinate plane.
Naturally, projections that are optimized for small areas have the advantage of having smaller distortions over those areas, but they still do have distortions that need to be accounted for in the highest quality surveys of significant extent. That would not include surveys of house lots in small subdivision.
The obvious advantage of the small area projections are that because the map projection scale factor changes at such a small rate, over sufficiently small areas it isn't a completely unreasonable assumption, for much routine work, to assume that the map projection scale factor is constant. That was traditionally convenient for computations before computers and programmed solutions were as completely available as they are now.
Even in many of the earlier generation data collectors, the map projection scale factor could be entered as a constant for the job and the reductions of measured distance to grid could be done from the computed ellipsoid height of the instrument based upon the survey measurements. The height scale factor is so insensitive to small errors of ellipsoid height that in virtually any traverse minor height misclosures had zero effect on the reduction of measured distances to the ellipsoid.
But, in any event, the question of the relation of the topographic surface to the reference ellipsoid is just that, something that has to be worked out independently of the map projection to get a rigorous solution.
What the typical LDP "solution" does is to throw in a one-size-fits all scale fudge factor to keep from having to exactly deal with the effects of topography.
> Inasmuch as a properly designed and implemented LDP will have a DEVELOPED SURFACE "far" from the Ellipsoidal Surface (which of course is one of the advantages of an LDP), the "Elevation Factor" (which I prefer to call a vertical coefficient or 'vc')
Well, you can call it the Jay Factor or the Ray Factor, but it's still the Height Scale factor since it is a scale factor that depends entirely upon the height of the station to which a horizontal measurement refers and has nothing to do with the fundamental map projection onto which the fudged Height Scale factor is grafted.
As the LDP bus hits the downhill run, everything that I predicted years ago is coming to be and in spades. The latest website where you put in a quarter and get an LDP (or rather a set of numbers that you can tell the software in your data collector to make sense of) for the project of the day is more bizarre than I could have ever imagined.
hi kent
> i sent you an email moments ago, i need a clarification from an earlier post.
For the record, I never got an email from you. Did you use the wrong longitude sign convention? :>
Distortion IS
Kent,
care to explain to those of us in the ignorant age who don't understand geodesy or projections, why there is an apparent contradiction in your age old understanding of distortion and what the actual words defined in your link and the Stem manual say?
Sure would be helpful.
Distortion IS
> care to explain to those of us in the ignorant age who don't understand geodesy or projections, why there is an apparent contradiction in your age old understanding of distortion and what the actual words defined in your link and the Stem manual say?
Sure. There are of course several different possible distortions inherent in a map projection. What is under discussion is scale distortion since the projections used in the SPCS are all conformal, not subject to angular distortion. Distortion in this sense refers to the change in projection scale across the mapping area. That is exactly what both my link and the NGS Manual mean. In the case of the NGS Manual they are referencing the change across the entire projection. In small areas, obviously the change in projection scale is much less.
The classical problem was of projecting the entire surface of the earth and that was where the problem of characterizing distortion began, i.e. characterizing the lack of uniformity of scale or of preservation of azimuth or angular relationships.
Distortion IS
"That is exactly what both my link and the NGS Manual mean".
Just not what either actually says. Both discuss scale distortion everywhere except the standard lines. Distortion is quantified by the scale factor. It has nothing to do with rates of change in the scale factor. What I've seen from both texts make no mention of change in scale factor, only the scale factor itself as the distortion.
Distortion IS
> "That is exactly what both my link and the NGS Manual mean".
>
> Just not what either actually says. Both discuss scale distortion everywhere except the standard lines. Distortion is quantified by the scale factor. It has nothing to do with rates of change in the scale factor. What I've seen from both texts make no mention of change in scale factor, only the scale factor itself as the distortion.
Sorry, that is missing the big picture. There are different measures of scale distortion in a map projection, but all of them I'm aware of take into account the change in projection scale factors across the mapping area. It's true that there are different ways of quantifying the distortion of the projection, but all of them consider more than scale at just one point.
> Kent's weird and indefensible position was that a distortion is only the rate of change in scale factors.
That's fun for you, I'm sure, but what I actually posted was:
>Actually, the word "distortion" when applied to a map projection refers to the non-uniformity of scale across the area projected onto the developable surface. So, to characterize the distortion of a map projection, you describe how the map projection scale factor *changes* over the mapping area. - See more at: http://beerleg.com/index.php?mode=thread&id=297239#p297374
Which is how the distortions inherent in different map projections are actually characterized.
Naturally, projections with highly distorted scale will reflect that fact in the rate of change of projection scale factors.
hi kent
my longitiude sign convention was sending you an email through the beerleg link.
this was a request for clarification of my compass and chain retracement post: [msg=296394]clarification[/msg]
Apparently, it's only lately that an ignorant age wants to redefine distortion to mean SF doesn't equal 1.000000, which is patently absurd. - Kent
Everywhere else on the map, scale distortion exists, and is defined as the ratio of the map scale at a given point to the map scale along a standard line. The scale distortion is identical to the grid scale factor. - Stem
I'm often wrong. Just ask my wife. You were wrong on this, which is fine, except that you had to belittle those in disagreement. That's where the rub is. You can spin it however you choose.
hi kent
> my longitiude sign convention was sending you an email through the beerleg link.
>
> this was a request for clarification of my compass and chain retracement post: [msg=296394]clarification[/msg]
For some reason, I've never gotten email via beerleg. kentmcm AT swbell DOT net should reach me.
> You were wrong on this, which is fine, except that you had to belittle those in disagreement. That's where the rub is. You can spin it however you choose.
Sorry, but the point stands that when you want to talk about distortion inherent in a map projection, you have to examine the way that the scale factor changes across the mapping area. Just because the Small Area Projections (SAPs) deal with relatively tiny areas compared to the SPCS projections doesn't mean that you can pretend that SAP-related issues apply to all map projections.
hi kent
Mail through SurveyorConnect/Beerleg has worked from me to several other posters. If it doesn't work to you, then either you typed the wrong email address in your profile or it is going to a spam folder.