The True distance along the surface of the earth is considered to be the Geoid Distance correct? If i store coordinates of two points using a GPS without defining a geoid file, the distance between them sould be the elipsoidal distance correct. Only If i set correct geoid file, only then will i be getting the true geoid distance? Just want to make sure i'm getting this right.
On the other hand if i measure a distance with the traditional methods (measure tapes, EDM with total stations, lasers, leveling etc) and get a Horizontal distance, and also measure a GPS point somewhere nearby how do i convert to geoid distances?
The true distance will be dependent on the elevation. The difference between the Geoid distance and the Ellipsoid will be so small I would consider it to be negligible.
What you are looking for is the scale factor at the elevation you are working. OR, if you live and work close to sea level, just ignore me and carry on.
James
therock003, post: 403027, member: 12282 wrote: The True distance along the surface of the earth is considered to be the Geoid Distance correct?
The distance between 2 points along the arc of the earth, is geodetic distance. So I believe that is the word you are looking for.
Non corrected slope dist is not geodetic.
Think concentric arcs, at various radii. Thats elev factor.
Yes...your terminologies are a bit mixed up. The true distance between two points on the surface of the earth would be the distance along the ground up and down the slope(s). (example, if you are looking at mile-markers along a highway, that is the distance as driven on the road including curves and changes in elevations, etc. you gps between markers would not give an accurate result) The geodetic distance would be reduced to a common elevation but at the elevation you are working at and not reduced down to the ellipsoid. If you are close to sea level, that difference is negligible but if you are or more than 2 or 3 thousand feet above sea level, your difference is not as negligible. If you are boundary surveying you want to retrace based on your knowledge of how it was done when it was originally surveyed.
Okay, I'm not saying much more than the others. And it gets more technical than that. But just putting it into my own words.
I'll add my version, which doesn't really disagree with any of the previous.
Distances are usually either
-on the ellipsoid,
-actual horizontal on the surface,
-or mapping distance in some projection such as State Plane or a Low Distortion Projection.
Distance on a geoid is pretty much irrelevant but very close to that on the ellipsoid.
Inversing between GPS coordinates gives distance on the ellipsoid. The elevation factor gets you to ground distance. A projection scale factor gets you to distance on that mapping grid.
Bill93, post: 403059, member: 87 wrote: I'll add my version, which doesn't really disagree with any of the previous.
Distances are usually either
-on the ellipsoid,
-actual horizontal on the surface,
-or mapping distance in some projection such as State Plane or a Low Distortion Projection.Distance on a geoid is pretty much irrelevant but very close to that on the ellipsoid.
Inversing between GPS coordinates gives distance on the ellipsoid. The elevation factor gets you to ground distance. A projection scale factor gets you to distance on that mapping grid.
I agree with everything you said but I think what the original poster is missing is this. Plugging in a geoid to your data collector does not get you horizontal distances on the surface. It really has no effect on the distances. The combined scale factor does that.
OR, I just totally misunderstood the question!
James
therock003, post: 403027, member: 12282 wrote: The True distance along the surface of the earth is considered to be the Geoid Distance correct?
Make sure you talk to the dealer that sold you your equipment and software. different companies handle this information differently.
You might also consider going to a seminar, taking a class or reading some in depth material. you will find some good information https://www.e-education.psu.edu/geog862/node/1819&apos ;">HERE and elsewhere on the internet.
Picture help, too:
And above all: make sure you include the meta data with your report; so the recipient understands what you did, how you did it and what your results represent.
Have a great day, I know I will!
Dougie
Nobody mentions my favorite distance, which is mark to mark (which is a slant range). It is the only one (I think) that is unambiguous. Even an ellipsoidal distance depends on which ellipsoid is used. There are an infinite number of "horizontal" distances, depending on elevation. And of course grid distances depend on the projection and ellipsoid. A "ground distance" could be at the low point height, high point height, at the mean height, or anywhere in between.
I archive my data as mark-to-mark distances along with mark-to-mark zenith distance (angle). Invariant.
Actually I think you are correct even though the definitions used in the post are a bit confusing.
If you use a file that only has ellipsoid heights your ground distance will be slightly different than if a geoid height file is applied to it.
Here is an example:
inverse between two points with a geoid file applied to the data 21,328.94 us ft.
inverse between the same points without a geoid file 21,328.92 us ft.
these points are about 4000' in elevation, the geoid height is about 50 feet.
John Hamilton, post: 403125, member: 640 wrote: Nobody mentions my favorite distance, which is mark to mark (which is a slant range).
I didn't mention the mark to mark distance, but did mention the actual distance along the ground. That would be the actual distance you walked or drove if you went straight from one point to the other which would include the vertical curve you walk long the terrain.
John Hamilton, post: 403125, member: 640 wrote: Nobody mentions my favorite distance, which is mark to mark (which is a slant range). It is the only one (I think) that is unambiguous. Even an ellipsoidal distance depends on which ellipsoid is used. There are an infinite number of "horizontal" distances, depending on elevation. And of course grid distances depend on the projection and ellipsoid. A "ground distance" could be at the low point height, high point height, at the mean height, or anywhere in between.
I archive my data as mark-to-mark distances along with mark-to-mark zenith distance (angle). Invariant.
The distance by GPS that I take to heart is the ECEF inverse. Which is mark-to-mark chord and height difference. Which should closely match slope distance by EDM. Then, computing geodetic or grid.
The confusion over WHAT a ÛÏHorizontal DistanceÛ IS, never ceases to amaze me. There are any number of Right & WRONG ways to look at the issue.
For example, the practice of returning a Measured (Grid) Bearing & Distance annotated over a Record (GLO/BLM ÛÏTrueÛ) Bearing and Distance, always struck me as an Apples & Oranges ambiguity.
I realize that MANY folks spend their entire career without seeing a 1000 foot height difference in a given project, so some of this ÛÏstuffÛ is pretty esoteric to them. Those of us in the more mountainous areas of the World have to deal with the various nuances between the many definition of ÛÏhorizontal distanceÛ (Apples & Oranges & Pears & Plums & Apricots & Grapes & Kumquats...) pretty much on a daily basis.
Further confusion emerges when someone doesn't understand EXACTLY what their equipment is burping out, or exactly what their software is doing with that data behind the curtain.
If you have 2 stations (top & bottom) a mile apart, but varying in ÛÏelevationÛ by 1000 feet, and use a Total Station to measure the distance between them, you will get the same ÛÏslope distanceÛ between them, BUT the Total Station will burp out;
A horizontal distance of ~5279.87 (bottom to top)
A horizontal distance of ~5280.13 (top to bottom)
If you average the Vertical Angles (not the zenith angles) top and bottom, you will get 5280.00!
Which one is RIGHT? Well obviously IT DEPENDS on the definition of ÛÏHorizontal DistanceÛ that you are aiming for.
Of course as distances and relative heights increase, the variances increase.
Combined Average Factors and Low Distortion Projections can only minimize these variances within a given project, never eliminate them entirely (although pretty close out in pancake land). In the case above, you just have to decide where the ÛÏsweet spotÛ (most important/valuable piece of real estate) IS, and live with the fact that the Earth is roundish.
When Project relief is in the 3000 to 5000 foot range, things can get pretty dicey (pretty fast), if you don't pay attention to geometric realities.
Loyal
therock003, post: 403027, member: 12282 wrote: The True distance along the surface of the earth is considered to be the Geoid Distance correct? If i store coordinates of two points using a GPS without defining a geoid file, the distance between them sould be the elipsoidal distance correct. Only If i set correct geoid file, only then will i be getting the true geoid distance? Just want to make sure i'm getting this right.
On the other hand if i measure a distance with the traditional methods (measure tapes, EDM with total stations, lasers, leveling etc) and get a Horizontal distance, and also measure a GPS point somewhere nearby how do i convert to geoid distances?
Be cautious. The actual difference between GPS coordinates is an XYZ vector and is a straight line. That line can be reduced to PSC coordinates and elevations, but that is not the typical output. Because it is an XYZ value it also must be understood to be a slope distance. For all practical purposes you can, and I have reduce it to a flat surface distance by squaring it, subtracting the square of the difference in elevations and getting the square root of the that value. I compare that flat distance with the horizontal distance in my traverse observations and is typically the first check I do of my GPS with my field work.
Paul in PA
I'm pretty sure that he is asking if the geoid file makes a difference in what his horizontal distance is.
Geoid heights are generally between 0 and 100 some feet.
I don't see them get much over 50 feet or much below 40 feet.
Each 20 feet of elevation change makes about 1 ppm in the scale factor of a projection.
Thus a geoid file creates between 0 and 5 ppm, not very much actually.
Being really anal about numbers seems to be a function of being a surveyor, but using the ellipsoid height or the elevation makes very little difference in the horizontal distance, to me it makes sense to inverse between points with an elevation and not to points with the ellipsoid height which is just a modeled number after all.
Of course if you set a projection by using a scale factor (LDP), or use a state plane or other canned projection, then it doesn't make any difference if a geoid file is applied, you will get the same answer regardless.
All of the above information is on point and all of you guys really helped clear much of the confusion I encountered while researching all of this information online.
Actually most of my work is to be done on an island where i plan to practice surveying, in which distances are relatively small and elevations are close to sea level, so geodetic appliances are just a matter of theory but i feel as if its my duty to know what is really going on regardless on whether i will be make any real use of it.
My Next points
-If setting a geoid, when defining a project on the Data Collector software, really doesnt affect horizontal distances i'm thinking if not setting one at all and get elipsoidal heights, and only if and when needed, transform them in the office. I guess using a software like TBC lets you import 3d coordinates, and then setting a geoid, will transform those elipsoidal elevations to geoid. Will that be a good and safe practice (I mean storing elipsoidal elevs when doing field work).
-Is combined scale factor and Elevation Factor the same thing? Is there a way somehow to get that value (maybe NGS, but for worldwide positions) by entering the coordinates of a known point somewhere on the center of the site area i will be working, rather than doing a site calibration and occupying 3 to 4 control points trailing on the mountains?
RADAR, post: 403067, member: 413 wrote:
And above all: make sure you include the meta data with your report; so the recipient understands what you did, how you did it and what your results represent.
Have a great day, I know I will!
Dougie
Here on Greece we dont present clients with meta-data, just the finished product (plans). I'd like to know more of how you do that report-genaration and maybe i'll practice that on the future.
therock003, post: 403237, member: 12282 wrote: All of the above information is on point and all of you guys really helped clear much of the confusion I encountered while researching all of this information online.
Actually most of my work is to be done on an island where i plan to practice surveying, in which distances are relatively small and elevations are close to sea level, so geodetic appliances are just a matter of theory but i feel as if its my duty to know what is really going on regardless on whether i will be make any real use of it.
My Next points
-If setting a geoid, when defining a project on the Data Collector software, really doesnt affect horizontal distances i'm thinking if not setting one at all and get elipsoidal heights, and only if and when needed, transform them in the office. I guess using a software like TBC lets you import 3d coordinates, and then setting a geoid, will transform those elipsoidal elevations to geoid. Will that be a good and safe practice (I mean storing elipsoidal elevs when doing field work).
-Is combined scale factor and Elevation Factor the same thing? Is there a way somehow to get that value (maybe NGS, but for worldwide positions) by entering the coordinates of a known point somewhere on the center of the site area i will be working, rather than doing a site calibration and occupying 3 to 4 control points trailing on the mountains?
Here on Greece we dont present clients with meta-data, just the finished product (plans). I'd like to know more of how you do that report-genaration and maybe i'll practice that on the future.
Use the geoid model, ellipsoid heights are only a very crude representation of the earth's surface, like a stick figure drawing of a human.
On an island, which is probably a mountain top sticking out of the water since it's near Greece, it's even more critical.
At 39å¡N 22å¡E, the geoid is 33.3 or 33.2 meters above the ellipsoid, depending on which geoid model you use (those are two different NGA files, the precise numbers are 33.214 and 33.302, and the conversion to boldatni may have changed it by a millimeter or two). You'll probably want to use a geoid file published by the Greek government. The elevation factor at elevation 0 (which means geoid level, which may differ a few centimeters from sea level) is 0.99999479 using the more recent and finer NGA model. So if you forget to enter a geoid file and you take GPS readings converted to Greek state plane coordinates and ground measurements with gun and prism pole, you will be off by 5 ppm converting between them.
Here in North Carolina, if there is a geodetic monument within 2000 ft (610 m), we must show the bearing and distance to it on a plat. I show the monument even though it is farther than that. I have a GPS system, so I went to the monument, checked that the coordinates are reasonably close to the published coordinates, and took many GPS readings on the site. The distance to the monument is shown in both grid and ground distances. All other distances on the map are ground distances.