View Full Version : California Coordinate System circa 1978
10-11-2004, 12:15 PM
I have a State Highway Monumentation Map from 1978 that says "coordinates, bearings and distances shown are based on the California Coordinate System, Zone 3". I have just been in the office for a year, but have been in the field for 18+ years so I am learning every day. My question being is there a scale or combination factor I need to apply to the info on this map to obtain distances that I would physically measure on the ground. I have used these types of maps before in my field work and coordinated points for searching and I seem to remember using a scale factor, but I can't find any information that would help me. I am familiar with the 1992 adjustment of NAVD '83, and am comfortable moving between GRID and GROUND distances on maps based on this.
Good question. And you're not the only one that is learning everday.
Yes, similar to the NAD '83 values that you're familiar with, there is a combined grid factor (CGF) that you must apply to the grid values to get accurate ground distances.
Since your map was done prior to the NAD '83 Adjustment, the coordinates should be NAD '27 horizontal values.
The procedure for calculating ground distances from the coordinate inverse is essentially the same as the NAD '83 that you're familiar with.
To calculate an accurate ground distance, you must first determine the average combined grid factor to *DIVIDE* your grid distances by. Depending on your location within the zone, the CGF is usually less than one (1), resulting in grid distances being shorter than ground. If your project is at the north or south limits of the zone and near sea level, then the CGF will be greater than one, resulting in grid distances being longer than ground.
The CGF is the product (i.e., muliltiply) of the scale factor and the elevation factor. The scale factor varies with the latitude of the control point within the zone, while the elevation factor varies with the elevation of the control point. The procedure for determining the elevation factor in NAD '27 is a little different than NAD '83, due to the two different reference elipsoids that are used. NAD '83 elevations must be corrected to the elipsoid height for best results (usually about a 30 meter vertical difference in the contiguous U.S. from NAD '27 elevation factors). Once you have the CGF values for your control points, then you can take the average. There is some good software available for the HP48 that will do conversions and inverses for you, or you can calc 'em yourself with the help of the SPC projection tables. I think NGS has a couple of on-line programs that will do this function as well. (NADCON maybe?) If this sounds too complicated, it's really not, and if you're not licensed yet, it's real good practice for the LS exam.
Maps based on SPC values sometimes provide an average combined scale factor to faciliate the conversion. Generally, the smaller the project, the more accurate the published value will be. If your map doesn't have this, then you will need to do the legwork to figure it out.
I think NGS may publish some good reference materials that describe this process in detail.
If all I've done is confuse you, feel free to send me a private message if you wish and I will try to assist you.
Berk Blake, PLS CA
10-12-2004, 12:23 PM
Most of the CalTrans maps that I have show their grid factor on the face of the map, usually each page. In lieu of this factor, why not call the district office of CalTrans and see if you can talk to somebody in their survey office. Their combined factor should be in their files and it will be the correct factor to use on their map
Berk's suggestion is definitely the smarter way to go about this. When retracing the right-of-way work, it's best if you have the grid factor the surveyors actually used, even if it's proven to be incorrect (precise vs. accurate?). If the project extends several miles in a north-south direction and/or is mountainous there will be significant differences in the CGF depending on location. That makes it more difficult to arrive at the same project average CGF that the state did. If the highway dept. followed proper procedures though, your calculated average CGF should be close enough.
I've known some surveyors who will tie some of the control and then compute their own scale factors based on the difference between what they measured and the grid inverses. You're at the mercy of the quality of the work that was done previously when doing this, so I don't recommend it for real precise work.
I've also known a few surveyors who don't worry about the grid factor because they don't think it makes much difference. If you're lucky enough to work primarily around one of the zone's standard parallels and you are close to sea level, then the grid factor is real close to one, and you don't have to worry too much about it. It's easy to get complacent though, and when you work somewhere else where the factor is significant, you have to be prepared to deal with it.
If you are unable to get the Grid Factor from Caltrans, you could post a couple of your coordinate values and their respective elevations. I'm sure that someone would be happy to determine an acceptable CGF for you.
10-12-2004, 02:54 PM
Now that's service!
10-13-2004, 12:56 PM
I have since obtained all sheets to the set hoping the CGF was on the front sheet, but alas it wasn't on any of them. The highway map that I remember using in the field years ago had the cooridinates on the map in CA COOR but the distances labeled were ground distances. I picked my longest line that was both labeled by distance and coordinates and firgured a scale factor and checked it to my next few longest lines. On the map I have now the cooridinates and labeled distances match each other. I am going to try the district office first, then if I still can't figure it out I'll post something on this thread.
THANK YOU EVERYBODY, I am really happy to have found this forum and will continue to use it (lots of great info)
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