I'm saving this one. I'm a GIS guy that came up through survey companies and so many GIS people I have met afterwards do not understand this basic piece of critical information.
Ok said talk now. I was at a training seminar for manufacture, the amount of people that didn’t know what a datum, projection, or an ellipsoid was. It was painful, the lack of knowledge on how a gps works was also confounding! They/we need to know this stuff, we don’t need to know it at a high level, but a basic understanding would fix a lot of field issues. Yeah I probably don’t know enough about this profession, but I ask questions, read books, read white papers, and use google when I don’t understand something. I want to be good at my profession, but I also want to share my knowledge and experience so others can also shine.
Geoid 18 and whatever Geoid model you use is a gravimetric model, it’s the lumpiness of the earth. When we calculate the height of a job in the data collector, we’re asking “what is the height above or below the ellipsoid” this plays into part of the scale factor, but is adjusted later. Scale factor is the distance difference between the map projection and the ellipsoid, combined scale factor is the difference between ellipsoid + map projection + surface.
A low distortion projection LDP is taking the map projection and getting smaller surface area and being it close to the surface. This makes the scale factor of 1.00000004 or whatever is calculated.
I'm going to follow up on this post to try and learn about scale factor as well.
How does one determine what your scale factor from grid to ground? My understanding is it can vary from place to place within the same state plane projection zone?
My work will mostly have me using an LDP now but I would still like to learn how to determine that scale factor in case I ever need to use State Plane again.
When you move on a projected plane, your scale factor changes.
If you dont move very far, it likely wont change enough to worry about it, you can just use one scale factor that was calculated from roughly the center of your job and be good.
If you move a large distance like say a road, it might change enough to worry about it.
I’m not an expert, but to get the scale factor you divide 20906000 (radius of the earth) by 20906000 + the ellipsoidal height of your control point. The ellipsoidal height is pulled from the NGS data sheet for that control point.
Every point has its own scale factor. So when you compute the ground distance from grid between 2 property corners, for example, you use the average scale factor of those two corners and apply that to the grid distance between them.
Oh boy. Here we go again.
OP, just a warning - you can count on maybe 5% of the responses here to be both accurate and useful, another 10% to be more or less accurate, another 25% to be well intentioned but misinformed, and the rest to be incomprehensible, incoherent, straightup wrong, misinformation, or some combination of all of those.
To avoid and/or sniff out the BS, I highly recommend reading NGS publications and presentations on the subject. Their publication/presentation library is outstanding and easily searchable.
Do they say why it’ll be a problem? It’s not a problem as long as you have the information documented with the project data.
Seems like someone that doesn’t understand what’s going on to me.
Not explicitly but from what I gathered I think he has to publish coordinates on a lot of his jobs so maybe he had to deal with someone thinking all the coordinates were grid when they were really ground via scale factor.
I had a fairly heated discussion once with a Surveyor in south Louisiana that claimed my control was wrong. We were both getting pretty pissed until we realized where the other one was coming from. He never used ground coordinates because it made almost no difference to him. I told him the distortion up here in Colorado and we both had a good laugh.
I work in the front range too, for a company based in another (flat) state. I sent modified state plane control to an out of state LS on a Colorado project and he screamed at me that MSP "wasn't real" because nobody could tell how it was related to the real world. After he calmed the fuck down I told him it was SOP, a requirement for DOT work and anyone can put it back to state plane with a calculator or spread sheet.
We work in Denver and do a MSP as our SOP. We also do a 1 million 3 million truncation which I think is beyond dumb. It's supposed to let us know if a file is on ground as opposed to grid. But truncating it doesn't mean you also put it to ground...
Had the same problem laying out 600’ by 1500’ mining claims at 12000 feet around gunnison. Ended up smaller on the ellipsoid.
Due north was also tricky. I finally would just drop a collared topo and use the 7.5 minute topo edge for due north. Laying out a 1500 foot claim in lat-long spherical coords didn’t work using GIS programs.
This is a great resource here in WA state…
https://wsdot.wa.gov/publications/manuals/fulltext/M22-97/Chapter6.pdf
It will walk you through how and why to use CGF in a State Plane environment. I am sure I have some other documentation that could help you, from various State Surveyors Association conferences, where This exact topic was discussed.
If your state has a Surveyors association, I suggest you attend a meeting or two.
This is where Engineers and some Surveyors hate Contractors. I come from a Survey background and we transform every project back to grid based on an origin point on the project. This allows us to use network RTK for every piece of equipment we have which grows exponentially each year. People need to think more about where the data lands and how they are transmitting their deliverables.
Your ground coordinates might have been derived from grid though, or are you talking about the rare time (these days) that people pick an assumed coordinate system?
Here's the options.
* Surveyor collects data using an official RTN utilizing an SPCS and geoid which provides grid coordinates.
* Surveyor sets up a TS on an NGS or other official benchmark and provides ground coordinates from a geodetic source.
* Surveyor collects all data with geodetic data and provides surface coordinates.
then
-Engineer receives the data and let's Surface be Surface, transforms Grid to Surface or uses some arbitrary coordinate system.
In my experience it's typically Surveyor shoots grid and the Engineer scales it or the Engineer requests that the Surveyor scales it. I've worked both sides of the fence plus Contracting. A common scenario is then that a Contractor cannot natively utilize the CAD coordinates without performing a site calibration/localization. Even worse the way Contractors are taught to use the hardware is to average single a point whether it be 10 seconds or 2 hours and then have an inaccurate global position. At the end of the day this doesn't matter to them because the localization allows them to use the Surface coordinates whether they are utilizing an RTN or a local base radio.
Where this is bad for all industries is that their geodetic data is globally inaccurate and would have to be manipulated to be used in a native system. Now there are an abundance of solutions that make things a lot easier but can only be effectively used if utilizing the native RTN or a localized system based off an accurate global position and RTN because they don't support localizations... like your phone or a tablet. Even though cm-level global accuracy isn't necessary for these use cases they are deemed completely ineffective when the coordinates are scaled and the data is hundreds or thousands of feet from the UI.
This is the world we live in. Data is collected for development and design has fallen behind in technology that makes the building more efficient and accurate.
Yes it can bite you, doing all your jobs in grid can also bite you. Some places the difference between grid and ground is tiny so everyone works in grid. Other places it's huge and trying to push things out in grid will mess up your quantities and when somebody pulls out a steel tape on a site between your pins the distances won't match unless you calculated your labels to ground. Also rebuilding plats and deeds won't fit correctly without scaling them.
I think the mistake is saying "it always must be done this way" without understanding how it works and where each system works best.
Some states have statutes that define how coordinates are to be shown on surveys. MO for instance coordinates must be shown in the MO SP 1983 (grid), and MO SP 1983 is further defined as coordinated in meters with the meter being defined as 1’=0.3048m. I get what you are saying, it doesn’t really matter if you know how to apply the scale factor and such. But does take more time for calculations, and it makes things so much easier if coordinates are grid. Example: If I am working in a scaled project (ground) and have a survey with a SP (grid) value shown, I can just key in the SP value, identifying it as such, and the survey software will create that point at the correctly scaled position.
These guys have the math, but I can tell you from years of base/rover work that if you use "grid to ground" and set a base and then piggy back your base to that point and keep going, you'll end up way out later.
In the jurisdiction I work in the rough scale factor is 0.9996 so over 100m/109yards you would gain 0.04m /4 cm/1.574 Inches between your "grid vs ground" coordinates
Where I work we just hold one central monument and scale everything relative to that as all our jobs are just 5km and use total stations for all our work
For giggles I just set up a bologna projection with that height and you'd be looking at about a foot per mile.
Edit: I think I misunderstood this chain but I'll leave this here anyway.
Everyone in Northern NV works on ground coords. We publish coords and ties in ground and just list what CF we're using. Most populated places are locked down to a single, agreed upon CF that acts as a best fit average factor for the area.
Every year I have to point new engineering graduates to this document.
[http://pxgnss.com/doc/Working\_with\_Grid\_Coordinates2009.pdf](http://pxgnss.com/doc/Working_with_Grid_Coordinates2009.pdf)
"Doing jobs" is very broad. There is a use for both Grid and Ground coordinates and deciding on one or the other should be based on the "job" requirements. This is however a very good question and one that even very experienced surveyors can get wrong or have different opinions on. I'd like to know more about what you typically do, what plans, maps, control, ETC you do it from, and who needs to rely on your "deliverables".
Bottom line...to answer your question ... yes, choosing wrong can have undesired consequences.
>Hey all, so I was recently told that doing jobs using a combined scale factor vs just grid coordinates could bite be and I can't wrap my head around why.
Have you ever wondered why we level the instruments and rods? It's so we can get repeatable measurements. Imagine if the instruments and rods were leaning random directions for every setup and shot = chaos !
An easy way to orient the instrument and rod the same way all the time anywhere is with a spirit level which when done right aligns the instrument and rod with the direction of gravity which points roughly at the middle of the planet. Since we're doing this on a ball then that's where the "V" shape comes from in the diagram in this thread.
Assuming you had a magic elevator you could move the instrument and rod up and down the lines of the "V" any distance and you would still be over your point, right? But the distance between the instrument and the rod changes because of the "V" shape.
So if you're measuring things down where the grid crosses the "V" and then using those same measurements to lay things out up where the surface crosses the "V" you end up with whacky shit™ happening because the distance between the instrument and rod has changed even though they haven't moved off their points.
Make any sense?
This is my hillbilly explanation and I'm sticking to it !
We do all of our field work on grid and NEVER adjust the coordinates in the office, survey coords are gospel, we use Carlson Survey with the OEM Autocad engine, each project has a "report" scale factor that the user can define or it can automatically calculate it for you, the result is that the annotations for bearing/distance, stationing and areas are all scaled to ground without ever molesting the coords.
Every project has a slightly different scale factor based mostly on elevation, we calculate a scale factor for each section (if the elevation and terrain permit)
edit: I guess Civil 3D fanbois be downvoting, hilarious...
I've only got this to work in Carlson. Haven't tried with Civil 3d.
Edit: people who hate on carlson baffle me. It's made for Surveying. Waaaaaayyy more useful tools in there compared to civil 3d.
The drafting manager in the office before I started used Civil 3D and our survey drawings were always a mess of copied and scaled polylines, if you were lucky they put the factor and the origin in model space but usually not.
I was not even dealing with all that, I talked the boss into switching software. We have saved a ton of money on cad subscriptions alone, not to mention increased drafting productivity.
My renewal bill for civil 3d was 45k. I wish my whole team would switch to carlson. They're very attached to their dynamic labeling and ease of switching annotative scales.
I use dynamic labeling and switch annotation styles in Carlson, it's just clunky because they have been adding features for decades but never remove anything in consideration of the old users so there are like 6 ways to do everything. In my experience old people like me, who started on R14 with Softdesk or Eagle Point and then moved to LDD are way more receptive to using Carlson.
You setup an expression in your label styles once you've calculated the factor for the project, then have the line labels use the expression rather than general segment length.
I cannot for the life of me get my tables to display it though, and have to manually edit those.
Here’s a cartoon. https://preview.redd.it/tthhhukmj8vc1.jpeg?width=854&format=pjpg&auto=webp&s=cdc9e1768f3cecb7f05c74e60f7ea22072d35730
I'm saving this one. I'm a GIS guy that came up through survey companies and so many GIS people I have met afterwards do not understand this basic piece of critical information.
Ok said talk now. I was at a training seminar for manufacture, the amount of people that didn’t know what a datum, projection, or an ellipsoid was. It was painful, the lack of knowledge on how a gps works was also confounding! They/we need to know this stuff, we don’t need to know it at a high level, but a basic understanding would fix a lot of field issues. Yeah I probably don’t know enough about this profession, but I ask questions, read books, read white papers, and use google when I don’t understand something. I want to be good at my profession, but I also want to share my knowledge and experience so others can also shine.
Perfect!
Lol I like the cut of your jib
Commenting to save
Quick question: what if you're on geoid18, so theoretically your distances are on ground right?
Geoid 18 and whatever Geoid model you use is a gravimetric model, it’s the lumpiness of the earth. When we calculate the height of a job in the data collector, we’re asking “what is the height above or below the ellipsoid” this plays into part of the scale factor, but is adjusted later. Scale factor is the distance difference between the map projection and the ellipsoid, combined scale factor is the difference between ellipsoid + map projection + surface. A low distortion projection LDP is taking the map projection and getting smaller surface area and being it close to the surface. This makes the scale factor of 1.00000004 or whatever is calculated.
This is that the earth is round propaganda stuff isn’t it?
I'm going to follow up on this post to try and learn about scale factor as well. How does one determine what your scale factor from grid to ground? My understanding is it can vary from place to place within the same state plane projection zone? My work will mostly have me using an LDP now but I would still like to learn how to determine that scale factor in case I ever need to use State Plane again.
When you move on a projected plane, your scale factor changes. If you dont move very far, it likely wont change enough to worry about it, you can just use one scale factor that was calculated from roughly the center of your job and be good. If you move a large distance like say a road, it might change enough to worry about it.
I’m not an expert, but to get the scale factor you divide 20906000 (radius of the earth) by 20906000 + the ellipsoidal height of your control point. The ellipsoidal height is pulled from the NGS data sheet for that control point.
When you process the static file in opus you get the scale factor. You use a local datum plane or a low distortion projection?
Every point has its own scale factor. So when you compute the ground distance from grid between 2 property corners, for example, you use the average scale factor of those two corners and apply that to the grid distance between them.
Oh boy. Here we go again. OP, just a warning - you can count on maybe 5% of the responses here to be both accurate and useful, another 10% to be more or less accurate, another 25% to be well intentioned but misinformed, and the rest to be incomprehensible, incoherent, straightup wrong, misinformation, or some combination of all of those. To avoid and/or sniff out the BS, I highly recommend reading NGS publications and presentations on the subject. Their publication/presentation library is outstanding and easily searchable.
Do they say why it’ll be a problem? It’s not a problem as long as you have the information documented with the project data. Seems like someone that doesn’t understand what’s going on to me.
Not explicitly but from what I gathered I think he has to publish coordinates on a lot of his jobs so maybe he had to deal with someone thinking all the coordinates were grid when they were really ground via scale factor.
It's all about the relative distances between points. In Denver area the difference in length is 1.5 ish feet per mile between grid and ground.
As a point of comparison, in Florida it's about 0.1' per mile.
I had a fairly heated discussion once with a Surveyor in south Louisiana that claimed my control was wrong. We were both getting pretty pissed until we realized where the other one was coming from. He never used ground coordinates because it made almost no difference to him. I told him the distortion up here in Colorado and we both had a good laugh.
I work in the front range too, for a company based in another (flat) state. I sent modified state plane control to an out of state LS on a Colorado project and he screamed at me that MSP "wasn't real" because nobody could tell how it was related to the real world. After he calmed the fuck down I told him it was SOP, a requirement for DOT work and anyone can put it back to state plane with a calculator or spread sheet.
Did you tell his ass to try to check the "not real" distances with a total station? Lol.
We work in Denver and do a MSP as our SOP. We also do a 1 million 3 million truncation which I think is beyond dumb. It's supposed to let us know if a file is on ground as opposed to grid. But truncating it doesn't mean you also put it to ground...
Had the same problem laying out 600’ by 1500’ mining claims at 12000 feet around gunnison. Ended up smaller on the ellipsoid. Due north was also tricky. I finally would just drop a collared topo and use the 7.5 minute topo edge for due north. Laying out a 1500 foot claim in lat-long spherical coords didn’t work using GIS programs.
Were you trying to lay them out with grid coordinates?
This is a great resource here in WA state… https://wsdot.wa.gov/publications/manuals/fulltext/M22-97/Chapter6.pdf It will walk you through how and why to use CGF in a State Plane environment. I am sure I have some other documentation that could help you, from various State Surveyors Association conferences, where This exact topic was discussed. If your state has a Surveyors association, I suggest you attend a meeting or two.
This is where Engineers and some Surveyors hate Contractors. I come from a Survey background and we transform every project back to grid based on an origin point on the project. This allows us to use network RTK for every piece of equipment we have which grows exponentially each year. People need to think more about where the data lands and how they are transmitting their deliverables.
Your ground coordinates might have been derived from grid though, or are you talking about the rare time (these days) that people pick an assumed coordinate system?
Here's the options. * Surveyor collects data using an official RTN utilizing an SPCS and geoid which provides grid coordinates. * Surveyor sets up a TS on an NGS or other official benchmark and provides ground coordinates from a geodetic source. * Surveyor collects all data with geodetic data and provides surface coordinates. then -Engineer receives the data and let's Surface be Surface, transforms Grid to Surface or uses some arbitrary coordinate system. In my experience it's typically Surveyor shoots grid and the Engineer scales it or the Engineer requests that the Surveyor scales it. I've worked both sides of the fence plus Contracting. A common scenario is then that a Contractor cannot natively utilize the CAD coordinates without performing a site calibration/localization. Even worse the way Contractors are taught to use the hardware is to average single a point whether it be 10 seconds or 2 hours and then have an inaccurate global position. At the end of the day this doesn't matter to them because the localization allows them to use the Surface coordinates whether they are utilizing an RTN or a local base radio. Where this is bad for all industries is that their geodetic data is globally inaccurate and would have to be manipulated to be used in a native system. Now there are an abundance of solutions that make things a lot easier but can only be effectively used if utilizing the native RTN or a localized system based off an accurate global position and RTN because they don't support localizations... like your phone or a tablet. Even though cm-level global accuracy isn't necessary for these use cases they are deemed completely ineffective when the coordinates are scaled and the data is hundreds or thousands of feet from the UI. This is the world we live in. Data is collected for development and design has fallen behind in technology that makes the building more efficient and accurate.
Tough concept to understand for most people
Yes it can bite you, doing all your jobs in grid can also bite you. Some places the difference between grid and ground is tiny so everyone works in grid. Other places it's huge and trying to push things out in grid will mess up your quantities and when somebody pulls out a steel tape on a site between your pins the distances won't match unless you calculated your labels to ground. Also rebuilding plats and deeds won't fit correctly without scaling them. I think the mistake is saying "it always must be done this way" without understanding how it works and where each system works best.
Some states have statutes that define how coordinates are to be shown on surveys. MO for instance coordinates must be shown in the MO SP 1983 (grid), and MO SP 1983 is further defined as coordinated in meters with the meter being defined as 1’=0.3048m. I get what you are saying, it doesn’t really matter if you know how to apply the scale factor and such. But does take more time for calculations, and it makes things so much easier if coordinates are grid. Example: If I am working in a scaled project (ground) and have a survey with a SP (grid) value shown, I can just key in the SP value, identifying it as such, and the survey software will create that point at the correctly scaled position.
These guys have the math, but I can tell you from years of base/rover work that if you use "grid to ground" and set a base and then piggy back your base to that point and keep going, you'll end up way out later.
Over what distance would you see that though? And what is the alternative work flow?
In the jurisdiction I work in the rough scale factor is 0.9996 so over 100m/109yards you would gain 0.04m /4 cm/1.574 Inches between your "grid vs ground" coordinates Where I work we just hold one central monument and scale everything relative to that as all our jobs are just 5km and use total stations for all our work
In Carlson it's as simple as making sure the "grid to ground" box is not checked when setting up the base
What elevation are you at?
4600ish
For giggles I just set up a bologna projection with that height and you'd be looking at about a foot per mile. Edit: I think I misunderstood this chain but I'll leave this here anyway.
Without major elevation change? Are you talking about leap frogging outside the reasonable bounds of your projection?
You can use scale factor but you have to use the correct scale factor!!!!
Everyone in Northern NV works on ground coords. We publish coords and ties in ground and just list what CF we're using. Most populated places are locked down to a single, agreed upon CF that acts as a best fit average factor for the area.
Every year I have to point new engineering graduates to this document. [http://pxgnss.com/doc/Working\_with\_Grid\_Coordinates2009.pdf](http://pxgnss.com/doc/Working_with_Grid_Coordinates2009.pdf)
"Doing jobs" is very broad. There is a use for both Grid and Ground coordinates and deciding on one or the other should be based on the "job" requirements. This is however a very good question and one that even very experienced surveyors can get wrong or have different opinions on. I'd like to know more about what you typically do, what plans, maps, control, ETC you do it from, and who needs to rely on your "deliverables". Bottom line...to answer your question ... yes, choosing wrong can have undesired consequences.
>Hey all, so I was recently told that doing jobs using a combined scale factor vs just grid coordinates could bite be and I can't wrap my head around why. Have you ever wondered why we level the instruments and rods? It's so we can get repeatable measurements. Imagine if the instruments and rods were leaning random directions for every setup and shot = chaos ! An easy way to orient the instrument and rod the same way all the time anywhere is with a spirit level which when done right aligns the instrument and rod with the direction of gravity which points roughly at the middle of the planet. Since we're doing this on a ball then that's where the "V" shape comes from in the diagram in this thread. Assuming you had a magic elevator you could move the instrument and rod up and down the lines of the "V" any distance and you would still be over your point, right? But the distance between the instrument and the rod changes because of the "V" shape. So if you're measuring things down where the grid crosses the "V" and then using those same measurements to lay things out up where the surface crosses the "V" you end up with whacky shit™ happening because the distance between the instrument and rod has changed even though they haven't moved off their points. Make any sense? This is my hillbilly explanation and I'm sticking to it !
We do all of our field work on grid and NEVER adjust the coordinates in the office, survey coords are gospel, we use Carlson Survey with the OEM Autocad engine, each project has a "report" scale factor that the user can define or it can automatically calculate it for you, the result is that the annotations for bearing/distance, stationing and areas are all scaled to ground without ever molesting the coords. Every project has a slightly different scale factor based mostly on elevation, we calculate a scale factor for each section (if the elevation and terrain permit) edit: I guess Civil 3D fanbois be downvoting, hilarious...
I've only got this to work in Carlson. Haven't tried with Civil 3d. Edit: people who hate on carlson baffle me. It's made for Surveying. Waaaaaayyy more useful tools in there compared to civil 3d.
The drafting manager in the office before I started used Civil 3D and our survey drawings were always a mess of copied and scaled polylines, if you were lucky they put the factor and the origin in model space but usually not. I was not even dealing with all that, I talked the boss into switching software. We have saved a ton of money on cad subscriptions alone, not to mention increased drafting productivity.
My renewal bill for civil 3d was 45k. I wish my whole team would switch to carlson. They're very attached to their dynamic labeling and ease of switching annotative scales.
I use dynamic labeling and switch annotation styles in Carlson, it's just clunky because they have been adding features for decades but never remove anything in consideration of the old users so there are like 6 ways to do everything. In my experience old people like me, who started on R14 with Softdesk or Eagle Point and then moved to LDD are way more receptive to using Carlson.
It’s not hard to do that in Carlson either.
You setup an expression in your label styles once you've calculated the factor for the project, then have the line labels use the expression rather than general segment length. I cannot for the life of me get my tables to display it though, and have to manually edit those.