I came here hoping you guys could give me some advice. I'm a GIS guy with a few years of GPS experience (primarily RTK, with some post-processing as well). But I am not a land surveyor. I have a GIS project that involves entering a distance/direction line from a survey plat (the survey was performed by BLM) into GIS. The coordinate system used in the GIS is NAD83, Alaska Albers Equal Area Conic (it's a standard projection within ArcGIS).
The line in question begins at a benchmark for which I have NAD83 coordinates (obtained from the NGS data sheet for the benchmark). The line bears North 33° 11' 00" East for a distance of 139.31 chains. I don't know all the details of the survey, but it seems that the surveyors occupied the unknown northeast end of the line with their survey-grade differential GPS to get the location at that point, and then later in the office computed the distance and direction from the benchmark to the survey point.
In the past when I've had to enter a distance/direction in GIS, I've used the ArcGIS COGO construction tools to define a ground-to-grid correction factor (derived from nearby north/south section lines) and then constructed a 2-point COGO line using the distance and direction shown on the survey plat.
However, today I heard from a colleague that, in a situation such as this, it is more appropriate to create a geodetic loxodrome (rhumb) line using the distance and direction shown on the plat, rather than doing it my typical way with a ground-to-grid correction and a 2-point COGO line. As a test, I entered the distance and direction of the line using both methods and the northeast ends of the two lines were 29 feet apart, using the same distance and direction for each of the two constructed lines. The southwest ends of the lines were snapped to the benchmark.
Logically, it seems to make sense to use a geodetic loxodrome line, as the survey was accomplished with GPS, and of course the surveyors did not need to be concerned with a map projection when doing the GPS survey. But I want to make sure I'm using the appropriate method.
So, what do you think? What would be the best, most appropriate method to use to get survey plat data into GIS? Should I continue to use a ground-to-grid correction with a 2-point COGO line, or should I use a geodetic loxodrome line?
Here's what ESRI has to say about geodetic loxodrome lines: http://resources.arcgis.com/en/help/main/10.1/index.html#//01m70000003q000000
My initial thoughts (it's early here, give me a break, haven't had coffee yet)...
I am not so sure a GPS vector is a rhumb line (I need to think about this when I am fully awake). There will be a forward and backward geodetic bearing... But I am not sure how ESRI might be handling/computing/reducing the data. I believe there needs to be more information shared.
You should post an image of the portion of the plat you are working on. Any statements regarding the (lat,lon), survey methods, bearing reported as geodetic, astronomic, state plane, would help to elicit the best advice.
Kevin is on the right track. We need more information. One question I have is about the NGS data sheet for the origin BM. Quite often the horizontal coordinates for bench marks are scaled and can be rather inaccurate. Give us the PID of the bench mark. Also are the endpoints of the line in the same datum?
Also, if this BLM survey is of recent vintage... I wonder if you may be able to contact the BLM and request a shapefile to confirm/support a method of entering your data into your GIS. I would contact the nearest BLM cadastral branch manager.
I would suggest further study of the following (preferably in a geodesy or land surveying text) and determining exactly how your software is handling the following:
You might consider purchasing the following as a desktop reference: Geodesy for GIS Professionals.
I noticed I had a typo in my original message. I stated the length of the line is 139.31 chains. That's wrong. It is actually 139.91 chains. It is identified as Kenai Fjords National Park and Preserve Boundary on the survey plat.
Here's the additional information that Kevin and John asked for:
=============================================================================
=============================================================================
I don't suppose you have the lat/long of point "A" as determined by the surveyors? I was just curious.
After looking through the ARC GIS references you provided, it would seem the correct method would be the geodetic loxodrome method.
John, no unfortunately I don't have the GPS-derived lat/long of Point A.
Just to add a bit more detail to the conversation, the purpose of the BLM survey was to locate and monument a 3-way shared boundary point between Chugach National Forest, Kenai Fjords National Park, and Kenai National Wildlife Refuge (Forest to the north, Park to the east, and Refuge to the west). Point A is the 3-way shared boundary point. It is also the northeast end of the line that extends from Benchmark Menker to the right bank of the Resurrection River.
When I constructed the line using the ArcGIS COGO tools, it put Point A in the middle of the river, rather than on the right bank. However, when I then used the ArcGIS geodesic loxodrome method to construct the line, it placed Point A on the right bank of the river, just as shown on the survey plat (I used a high-res satellite image as a basemap reference). So that's some pretty strong evidence that the loxodrome method is closer to the truth than the COGO method. But since I'm not a licensed surveyor, I wanted to get this forum's expert advice so that I have 100% certainty that I did it right.
Below are a couple screenshots of the two constructed lines. The first image shows the entire line from benchmark to the river. The second one is zoomed in on the northeast end (Point A).
======================================================================================
Well... based upon the data sheets for LAKE (PID UW7708) AND MENKER (PID UW7709) it would appear that the BLM is really reporting an averaged geodetic bearing. The calculated distance per the data sheet conversion would be 496.43 chains (BLM is reporting 496.47).
I put the coordinates for the control stations into the NGS tool INVERSE and received the following results:
Ellipsoid : Clarke 1866 (NAD27)
Equatorial axis, a = 6378206.4000
Polar axis, b = 6356583.8000
Inverse flattening, 1/f = 294.97869821380
First Station : MENKER
----------------
LAT = 60 17 1.35700 North
LON = 149 48 11.91900 West
Second Station : LAKE
----------------
LAT = 60 22 19.40800 North
LON = 149 46 22.19700 West
Forward azimuth FAZ = 9 41 38.5386 From North
Back azimuth BAZ = 189 43 13.8731 From North
Ellipsoidal distance S = 9986.6794 m
To me this would indicate that instead of entering the bearing and distance as a loxodrome between the NGS stations (which the BLM set a monument in the position of MENKER) you would enter this as a mean geodetic bearing if such an option exists in the ESRI software.
I would contend that the line run on the ground by the BLM is being shown at ground distances (probably on the mean elevation of the project). So this would have me considering the use of combined scale factor to bring the distance (only the surveyed line by the BLM not the tie between the NGS monuments) back to GRID.
That being said I am probably leaving you with more questions than answers. I would seriously consider contacting the BLM surveyor that conducted the survey or the chief that issued the special instructions. It would also be a good idea to touch base with an NGS geodetic advisor or the BLM geodetic contact to verify that you are approaching this correctly.
Kevin, thank you and the others for helping me with this. I knew I would get some expert advice here, even if I don't fully understand all of it!
My colleague at the National Park Service (the one who got all this started) has contacted the BLM surveyor for his advice as well. So we're hoping to hear direct from the source in a day or two.
The reason we want to make sure this is absolutely correct is that the northeast end of the line is a very important point. It marks the boundary between three different Conservation System Units, each with their own unique mission, management objectives, and regulations.
I'll post back here with a summary once we hear from the BLM surveyor.
srmak, post: 308540, member: 9682 wrote: Kevin, thank you and the others for helping me with this. I knew I would get some expert advice here, even if I don't fully understand all of it!
My colleague at the National Park Service (the one who got all this started) has contacted the BLM surveyor for his advice as well. So we're hoping to hear direct from the source in a day or two.
The reason we want to make sure this is absolutely correct is that the northeast end of the line is a very important point. It marks the boundary between three different Conservation System Units, each with their own unique mission, management objectives, and regulations.
I'll post back here with a summary once we hear from the BLM surveyor.
Did you ever find a resolution for this?
Have you checked with the BLM if they have GCDB'ed this area.
BLM uses mean true bearing and Alaska usually assumes distances are 'sea-level'. They have programs that will allow them to traverse directly entering mean true bearing. You can also do something similar to what you are doing, compute an approximate grid midpoint then compute the grid rotation angle and apply it. Check with NGS inverse program, otherwise you can get there with a few iterations of the direct program to get the mean forward and back to come out to the desired mean bearing. Then you can convert to your grid.
- jlw
srmak, post: 308325, member: 9682 wrote: I came here hoping you guys could give me some advice. I'm a GIS guy with a few years of GPS experience (primarily RTK, with some post-processing as well). But I am not a land surveyor. I have a GIS project that involves entering a distance/direction line from a survey plat (the survey was performed by BLM) into GIS. The coordinate system used in the GIS is NAD83, Alaska Albers Equal Area Conic (it's a standard projection within ArcGIS).
The line in question begins at a benchmark for which I have NAD83 coordinates (obtained from the NGS data sheet for the benchmark). The line bears North 33å¡ 11' 00" East for a distance of 139.31 chains. I don't know all the details of the survey, but it seems that the surveyors occupied the unknown northeast end of the line with their survey-grade differential GPS to get the location at that point, and then later in the office computed the distance and direction from the benchmark to the survey point.
In the past when I've had to enter a distance/direction in GIS, I've used the ArcGIS COGO construction tools to define a ground-to-grid correction factor (derived from nearby north/south section lines) and then constructed a 2-point COGlO line using the distance and direction shown on the survey plat.
However, today I heard from a colleague that, in a situation such as this, it is more appropriate to create a geodetic loxodrome (rhumb) line using the distance and direction shown on the plat, rather than doing it my typical way with a ground-to-grid correction and a 2-point COGO line. As a test, I entered the distance and direction of the line using both methods and the northeast ends of the two lines were 29 feet apart, using the same distance and direction for each of the two constructed lines. The southwest ends of the lines were snapped to the benchmark.
Logically, it seems to make sense to use a geodetic loxodrome line, as the survey was accomplished with GPS, and of course the surveyors did not need to be concerned with a map projection when doing the GPS survey. But I want to make sure I'm using the appropriate method.
So, what do you think? What would be the best, most appropriate method to use to get survey plat data into GIS? Should I continue to use a ground-to-grid correction with a 2-point COGO line, or should I use a geodetic loxodrome line?
Here's what ESRI has to say about geodetic loxodrome lines: http://resources.arcgis.com/en/help/main/10.1/index.html#//01m70000003q000000
jlwahl, post: 369698, member: 68 wrote: BLM uses mean true bearing and Alaska usually assumes distances are 'sea-level'.
- jlw
That computes very well on the Menker-Lake line. The distance is exactly the NAD 83 (1986) State Plane grid distance divided by the average scale factor, making it an ellipsoidal distance, the rough equivalent of a NAD 27 sea level distance. The grid az adjusted by convergence is
9d 41m 21.75s which is close to the published value, indicative of perhaps an average azimuth.
I wonder why the GIS didn't use Alaska State Plane?