Careful there, Mr. Moe...
Generally a data collector records the actual raw measurement. The coordinates produced may in fact be determined from a scaled distance, however, once that raw data is processed again, the scale factor needs to be applied again. The same is true for curvature and refraction. Even though this may be set in the gun, this only affects horizontal and vertical distances as displayed on the gun. The stored slope distance and vertical angle is as measured. Thus, when processed later, the curvature and refraction needs to be applied again.
Right on John.
Okay, I see what you are doing.
Yes leaving it in grid is fine, nothing wrong with that. Just say so on the map.
No biggie.
Well from the sounds of it, the scale factor is negligible. However, I would say that you use one "medium" or one surface. If you are running a total station using measured actual distances then GPS using distances at the SP Grid, then you are introducing a small error (or at least working in two different surfaces). The key is to try to reduce to one surface, in my opinion, and make it clear what your information (metadata) is. If you do everything on the "grid" then have your total station shots reduced to the same "grid". If you are working on real-ground surface, reduce your SP grid GPS data to the same surface.
Do what is consistent, and make sure your data collector, or internal total station software isn't reducing something that it shouldn't be reducing or causing a "double-reduction" between when you measure in the field and bring it into your computer.
It isn't for boundary work "now" and the engineers assure you that this isn't an issue, but I don't know how many times a survey or an old survey turns into doing a boundary on the same datum.
It isn't a fatal mistake as your colleague says (probably not even for boundary work at your elevation), but good practices and consistency is your friend.
There are several schools of thought on this, each with its own merits. Nearly every method is a valid tool depending on the job. I personally do most jobs on a predefined custom projection. The crew gets to work at ground and the unadulterated raw data is stored for post processing. At the end of the day the norm can and should be set aside if you are losing money and not gaining anything meaningful. Sometimes you leave the robot and GPS in the truck and drag out the compass and cloth tape. This one sounds like a prime case for holding one fixed and a bearing to the other. If the adjusted hits within a tenth or two move on...
:good: :good: :good:
I like being picky.There is no such thing as a"combined scale factor". You either have a scale factor or a combined factor.
Scale Factor and Convergence
Three optional output fields for Interactive and Point Database Conversions are Scale Factor, Convergence, and Orthometric Height Scale. These fields only apply when a projected coordinate system is chosen as the output system. Orthometric height scale will only display when a Vertical Reference is selected. Points in a Geodetic systems do not have a scale or convergence.
[Grid] Scale Factor
Grid Scale Factor, often simply called "Scale Factor" is a measure of distortion at a given point on a projected map. The scale factor is not cartographic scale, but a factor used to calculate actual ellipsoidal distances rather than distances on the projected surface.
Convergence
Convergence is the angle of difference in direction from Grid North to True North. This will vary across a projected coordinate system and can be used as a measure of accuracy of angular measurements at a given point on the map.
Note: In Transverse Mercator projected coordinate systems, convention is to specify the convergence angle from True North to Grid North.
Orthometric Height Scale
Also known as Elevation factor, Orthometric Height scale represents a factor of elevation that can be used to calculate geodetic distances above or below the ellipsoid (also known as reducing to the ellipsoid). This scale is determined using a constant radius for the earth in the area of the calculation, but this is typically considered accurate enough for most applications.
Combined Factor
Combined Factor is simply Grid scale multiplied by the Orthometric Height Scale. This factor is used to calculate ellipsoid distance from a grid distance above or below the ellipsoid.
Example: The Grid Scale for two points is 0.999689, and the Orthometric Height scale for the points is 0.999999123, the combined factor is: 0.999689 x 0.999999123= 0.999688123272747
If the grid surface distance between the two points is 1000 meters, to calculate ellipsoid distance between the two: 0.999688123272747 x 1000meters = 999.69 meters
This is right up there with most surveyors callin all jobs "topos". They are not.
Clarity in communications demands proper nomenclature. Plus ii makes you sound more professional.
(And no, I do not want to get into a discussion about "Major" and "Minor" contours. That is an AutoDesk thing and does not exist in the real world. If you don't know the proper names of the different types of contours, you don't need to be drawing no maps!!) 😛
Sorry if I'm confused, but I don't understand the concept of a "GPS traverse". It's either good or it isn't.
Total station closure, no problem. It's adjustable if you feel the need, but I generally don't adjust anything and just go fix a problem if I see one. Chasing a couple decimals around is borderline crazy.
GPS is 110% at the mercy of our sacred satellites, and if they have a kink - you have a kink.
Just go re-shoot everything and you'll sleep better. May even stand up in court....
Mr Dent,
I think you are wrong on one point. Every time you are referring to "Orthometric height" I think you mean "ellipsoid height". To calculate a distance from the ellipse surface to the elevation of your work you need the measurement from the ellipsoid. (I don't think it really makes a difference if you use the orthometric heights as to your elevation scale factor, (ie: close enough).
Combined Scale Factor bothers me as well. I think a surveyor should think in terms of the Grid Scale factor, and the ellipsoid scale factor as far as the "metadata" goes.
It absolutely makes a difference (ortho versus ellipsoidal height). In my area, where the separation is about 30 m (100 feet), it makes about 6 ppm difference.
An older version of Corpscon incorrectly used the orthometric height instead of the ellipsoidal height when computing the elevation factor.
The fact that the ellipsoid-geoid separation was not known in NAD27 (i.e. pre-satellite days) caused a systematic bias across the US. Here is a map that few have ever seen, the separation between the clarke 1866 ellipsoid (as used in NAD27) and "sea level":
Most of the east and central US had little distrotion from this, but you can see that as you go west it became quite large. Once they were able to compute this, they realized the magnitude of the bias. One of the various reasons to do the NAD83 adjustment.
Yes you should use it. The software makes it too easy to not use it.
That definition is straight out of the text book, I didn't write it.
> ...you state you get closures in the 30000's...you will get this whether or not you apply combined factors if the traverse is a closed loop...
That was my first thought on reading only the post title.
> the text book
is incorrect on that issue.
I wondered why Iowa had the biggest local anomaly east of the Rockies, and then remembered that was the area of the Manson meteor impact 74 million years ago, one of the largest in North America.
Wow, you have a long memory.
Perhaps that is why NGS will be doing their next Geoid Slope Validation Survey across Iowa.
Interesting that they chose to follow US30. I don't think they will find a lot of the old passive marks that they want to compare against within their 500 meter range of the new line.
The nearby 1930's level line mostly followed the major railroad that parallels US30, but is a few miles away for much of the length. My brother investigated all of those marks in the eastern 1/3 of their current project (and further east) and reported them to NGS when he was a track inspector for the railroad. He was supposed to check condition of bridges and culverts, so easy to also note if the old disk or rivet was there, although the RR didn't use the marks.
"I hate being picky but......"
Well, the difference is negligible. One other "picky" point I would make is I would not use the simple term "scale factor" at all. There are "grid", "ellipsoid", and "combined" scale factors. Of course "elevation scale factor" is a term that could cover both the NAD'27 's sf, and the ellipsoid sf of today.
As far as being negligible for survey purposes, so is the "grid" sf. We were being picky though. 😉
Knowing what you need is critical. Look at AutoCAD and Carlson, When you enter your factor it says in the software "scale factor". If you plug in a "combined scale factor" you are going to get noticeable errors. (The software factors in the point's elevation for adjustment. In essence, if you enter the combined factor, you double up on the elevation adjustment.)
