Survey Pro and GNSS Scale factors

If you are working in state plane coordiantes and using GPS, the choices don't really matter much as far as scale factors go. If you are mixing EDM shots, it does matter. You will notice when you change from GNSS to Optical in survey pro, the choices change when it come to scale factor. I usually pick "use a single combined scale factor" and pick a point in the middle of the job. This will basically keep everything on grid is my understanding. The scale factor is usually very small in my area, so the a combined factor usually works well.

For example the scale factor for the job I'm working on now is 0.9999369 for a point in the middle of the job. Another point on the project that is about a half mile away has a scale factor of 0.9999376. In this case, if I took a shot 10,000 feet long the difference between the two scale factors would be 0.007' I suspect the other choices would change the scale factor every so slightly depending on where you are on a job site. Maybe on a large site with a lot of elevation change, it would be more critical.

If you inverse while you are in the GNSS it will give you both a grid distance and a ground distance. I'm assuming thats using a scale factor for that area of the project.

When I do layout work, I usually just keep working in state plane coordinates if its a project we worked on. The exception would be if I'm doing a building. My procedure is to set two of the corners with the GPS, set up on one with a total station and adjust the distance to match. Then I make sure the scale factor is set to 1 and finish staking the building. Once again this works for me because I work in an area where the scale factor is very small.

Hope this helps.

Actually i'm operating in Europe and I'm on a Transverse Mercator Projection system that uses a 0.9996 scale factor. Whys shouldnt that be used instead of a locally calculated value?

And besides the info shown on inverse function, does the software handle transformation upon capturing a shot based on that value. n the case of GNSS your output is point coordinates so how is that value applied? Maybe it transforms the baseline between Base station and said point on the RTK in progress?

I work in Illinois which is also traverse mercator I think. I'm a little weak on explaining this stuff but my understanding is the scale factor changes as you move east and west in a zone and also somewhat base on your elevation. There fore there is no set scale factor for a projection. It changes even on a small site but the smaller the site, the closer it becomes to where a single scale factor can be use. Almost all the jobs I work on I just use on scale factor but they are usually no more than a mile square.

Your second question I'm not sure how to answer. If you are using GPS and working with a coordinate system, all the coordinates are on that system and in my case, plane coordiantes. In my case, I'm often working on boundaries so I want to show ground distances. This is where the scale factor becomes important.

therock003, post: 423423, member: 12282 wrote: Actually i'm operating in Europe and I'm on a Transverse Mercator Projection system that uses a 0.9996 scale factor. Whys shouldnt that be used instead of a locally calculated value?

And besides the info shown on inverse function, does the software handle transformation upon capturing a shot based on that value. n the case of GNSS your output is point coordinates so how is that value applied? Maybe it transforms the baseline between Base station and said point on the RTK in progress?

The Projection scale factor you mention is the scale reduction from geographic (on the ellipsoid) to the projected plane. There are two other factors to consider.
The 'scale factor' at each point takes you from the plane to a nomimal mean earth radius ground point. The 'elevation factor' scales you again to nominal ground based on your elevation.
The technically correct application is to scale grid (projected plane) distances by the average of the 'combined scale factor' at each end of the line. The more common practice is to adopt an average combined factor for the site and apply it to the whole coordinate.
There are a bazillion ways to do this, each driven by location, needs or just plain preference. Hope that helps, Tom

therock003, post: 423423, member: 12282 wrote: Actually i'm operating in Europe and I'm on a Transverse Mercator Projection system that uses a 0.9996 scale factor. Whys shouldnt that be used instead of a locally calculated value?

And besides the info shown on inverse function, does the software handle transformation upon capturing a shot based on that value. n the case of GNSS your output is point coordinates so how is that value applied? Maybe it transforms the baseline between Base station and said point on the RTK in progress?

Don't use .9996, that will not work. The origin point for your zone must be a long way from Greece. You would need to figure out just what the combined scale factor is locally, it may be if you are near sea level you should just work in the plane and not try to adjust.

I think you are using trimble? If so the easy way to get a scale factor is to put two points into your software and inverse them, you should see a ground distance and an ellipsoid distance. Divide the ellipsoid by the ground number and you will get the combined factor. You don't need real survey points you can imput points in the area in question by lat, long, ht. You can even do the same points with different elevations and get a feel for how the scale factor changes because of elevation changes.

Lee 0.9996 is my national mean grid to ground scale and i cant not use it as it is defined internally on the project settings.

Now of course there is an exact formula defined in the books, that calculates the exact scale factor given the center of the site of the project and i have calculated that value to be 1.0020 for the region i'm doing most if not all of my work.

So my question now becomes, how can i use that (1.0020 instead of 0.9996), should I key in zone and punch it here

and also which of the options of survey pro should be ticked in order to respect that value and disregard the 0.9996?

therock003, post: 423588, member: 12282 wrote: Lee 0.9996 is my national mean grid to ground scale and i cant not use it as it is defined internally on the project settings.

Now of course there is an exact formula defined in the books, that calculates the exact scale factor given the center of the site of the project and i have calculated that value to be 1.0020 for the region i'm doing most if not all of my work.

So my question now becomes, how can i use that (1.0020 instead of 0.9996), should I key in zone and punch it here

and also which of the options of survey pro should be ticked in order to respect that value and disregard the 0.9996?

The scale factor of 0.9996 is not a grid to ground factor. It is thr scale reduction applied to project to the utm plane.Scaling to ground is a separare task. It can be accomplished by editing the projection scale factor. I dont recommend that method until you get more comfortable with standard techniques.

I mistyped before you divide the grid distance by the ground distance to get the scale factor.

I did a quick imput and got these values in Greece along the eastern coast between two points using TM07:

Grid distance 13491.166m
Ground distance 13495.192m

this creates a scale factor of .99970167.

No doubt I'm not in the area you are in, but I can't imagine 1.002 ever being a valid number.

You'd be surprised by the range of that scale. Not sure where youre getting those grid/ground info, but if you'd punch in the following global coordinates (degrees decimal) i guess you'll find that what u'm saying is accurate

37.727686
23.487190

therock003, post: 423622, member: 12282 wrote: You'd be surprised by the range of that scale. Not sure where youre getting those grid/ground info, but if you'd punch in the following global coordinates (degrees decimal) i guess you'll find that what u'm saying is accurate

37.727686
23.487190

I created two points with a height for each one of 45m.
one at the above coordinates and one south at 37.70000

When I inverse them I get a ground distance of 3072.92m and a grid distance of 3071.747m

a combined scale factor of .999618

What method are you using to get those distances? The 0.9996 you are getting is the mean for all greece. Maybe your software is not taking into account the point-specific textbook formula and only uses that mean value to give you the grid/ground distances

I'm even certain that 10 years ago in the university, the professors told us that this is the scale factor in grid-to-ground distancing.

Okay, so I have been [more or less] following this discussion, and I have a question for therock003;

Are you trying to use the Hellenic Geodetic Reference System 1987 (HGRS87), aka the Greek Geodetic Reference System 1987 (GGRS87)?

Loyal

Yes Loyal that is correct

therock003, post: 424030, member: 12282 wrote: What method are you using to get those distances? The 0.9996 you are getting is the mean for all greece. Maybe your software is not taking into account the point-specific textbook formula and only uses that mean value to give you the grid/ground distances

I'm even certain that 10 years ago in the university, the professors told us that this is the scale factor in grid-to-ground distancing.

I opened TBC and set TM07 as the zone then put in your point and one south. Then I inversed between them to figure out my combined scale factor for that line. The origin point for TM07 is the equator and 24E longitude. Since the line I put in is almost along the 24E longitude line it's near dead center of the TM zone. Because I gave elevations very near sea level it's not surprising that the combined scale factor is close to .9996.

MightyMoe, post: 424061, member: 700 wrote: I opened TBC and set TM07 as the zone then put in your point and one south. Then I inversed between them to figure out my combined scale factor for that line. The origin point for TM07 is the equator and 24E longitude. Since the line I put in is almost along the 24E longitude line it's near dead center of the TM zone. Because I gave elevations very near sea level it's not surprising that the combined scale factor is close to .9996.

Just a fascinating discussion. The ellipsoidal distance from NGS Inverse is 3072.8987 meters, just short of Moe's calculated ground distance, as it should be.

The scale factor given by [USER=12282]@therock003[/USER] is not the average scale factor for Greece. It is actually the scale factor at the central meridian for the projection that he's using. It has the smallest numerical value of any scale factor on the projection. Using it to scale measurements even somewhat far from the central meridian will create significant errors. Note that slide 12 in this link shows exactly the numbers that he shows in post #8 above. Greece is far too large for an average scale factor for the entire country to be useful.

https://cddis.nasa.gov/metsovo/docs/Katsampalos_Kotsakis_Gianniou_HEPOS.pdf

Post #8 in the thread gives some insight into the question [USER=12282]@therock003[/USER] wants answered. I know nothing of surveying software, but it seems to me that these various options affect the values of coordinates computed from measurements or distances computed from coordinates. It would seem to me that a user would have to know two things: the nature of the output he wanted and how to structure the input so that he gets that output.

As to the first question in Post #3, a scale factor is pure mathematics. It is the output from a continuous function. In the transverse Mercator system, an infinitesimal change in longitude creates an infinitesimal change in a scale factor. In this case, the further you get from the central meridian, the further the scale factor gets from 0.9996. The professionals here can give better guidance than I can, but, if you're more than a few thousand meters from the central meridian, you likely need a different scale factor.

Well, based on what I can find on the Internet, HGRS87 (GGRS87) is a non-geocentric ‰ÛÏlocal‰Û coordinate system:

From Wikipedia:
‰ÛÏHGRS87 specifies a non-geocentric datum that is tied to the coordinates of the key geodetic station at the Dionysos Satellite Observatory (DSO) northeast of Athens (38.078400å¡N 23.932939å¡E). The central pedestal (CP) at this location has by definition HGRS87 coordinates 38å¡ 4' 33.8000" N - 23å¡ 55' 51.0000"E, N = +7 m.‰Û

And:
‰ÛÏAlthough HGRS87 uses the GRS80 ellipsoid, the origin is shifted relative to the GRS80 geocenter, so that the ellipsoidal surface is best for Greece.[1] The specified offsets relative to WGS84 (WGS84-HGRS87) are: ëÓx = -199.87 m, ëÓy = 74.79 m, ëÓz = 246.62 m.‰Û

The above information isn't as concise as it could be (which WGS84?), so for our purposes, lets just ASSUME that the 3 Parameter Transformation values are relative to the current WGS84 realization (WGS84[G1762]). This is probably NOT the case, in that the ‰ÛÏreference epoch‰Û appears to be 1987.0000, which would suggest the Original WGS84! Which means that the transformation from WGS84(G1762) to WGS84(original) would also factor in.

AND further ASSUME that your software has properly applied said "corrections" to any WGS84 coordinate values derived via GPS observations.

Every website that I have found that lists the HGRS87 Transverse Mercator Projection constants agree on the following parameters:

Central Parallel 0å¡ (origin)
Central Meridian 24å¡ E (origin)
Ellipsoid GRS80 (shifted to best fit Greece)
Scale Reduction 0.9996
False Northing 0.0 Meters
False Easting 500,000.0 Meters

So, using therock003 Lat/Lon posted above (37.727686å¡N 23.487190å¡E), I get:

N = 4,175,725.664(m)
E = 454,810.523(m)
k = 0.999 625 151
g = -0å¡18'50‰Û (gamma angle Geodetic to Grid)

Greece appears to have about 2918 meters of relief, so without ellipsoid height values (in the HGRS87 ‰ÛÏdatum‰Û), it's impossible to accurately compute ‰ÛÏcombined scale factors.‰Û Considering this magnitude of relief, the variance will be significant depending on where you are within the country.

For example:

On Mount Olympus
40å¡05'08‰ÛN
22å¡21'31‰ÛE
2925(m) Eh (2918 + 7)

N 4,438,544.025(m)
E 360,063.245(m)
k 0.999 841 058
g 1å¡03'26‰Û
ef 0.999 541 398
caf 0.999 382 479

So 1000.000 meters on the ground = 999.382 meters on the "grid."

It appears that 'k' (grid scale factors) can range from 0.999 600 to in excess of 1.000 125 near Corfu, and 1.000 148 near Orestias (and probably more in the extreme East and/or West parts of the country).

Loyal

At least with my Survey Pro there is a tab under the "Survey" menu group titled "Projection Calculator" This will give a scale factor for any given point. Maybe your coordinate system isn't loaded in your data collector? If not I think you can input the parameters and it will be in there. Most likely the parameters are the ones Loyal has listed above but I've never personally tried this.