Latest Contributions by KenLucas

I changed the coord sys of a blank arcmap session to geographic - north america - NAD27. I added the historic tif in - I projected the lat and long coordinates to 10 points around the perimeter of the tif & 2 in the center - I used those points as geo ref link pts, entering the lat & long cords into the input boxes of the link2. I added in a more recent topo and compared positions in the geo reffed historic toop with the same positions in the new topo. Now I measure about 300-400' discrepancy across the map, instead of the 1,000' error I got when I set the arcmap up with WGS84 coordinates, before geo reffing the historic tif. How can I tell if the discrepancy is due to error within the old PLSS and not just error between the geo ref tif and the new topo data? What would you expect?  My goal is to be able to use this geo reffed historic tif to geo ref some historic property parcels.  Ken ... View more

Melita,   I appreciate you and Jim for advising us. I'm still unclear of the procedure you are suggesting for setting up the arcmap layer properties, before I bring in any data. My choices seem to be: 1. select Clarke 1866, out of the geographic coord sys folder 2. select NAD27, out of the  north America - geographic coord sys folder If one of these is not right would you, please, specify how I set up the layer props coordinate system? I like your suggestion to project lat and long coords to the, less distorted, center area of the old map. I assume I should determine the lat and long for 4 link points near the center of the map before geo referencing. Ken ... View more

Jim,   I apologize. I didn't comprehend your response closely enough. I just changed the arcmap properties coordinate sys to Clarke 1866, as you suggested, and the map units are now in lat and long. Is this what we should use, if we enter the lat and long cords into the link x y input? Or would it be more effective to link to map features?  We realize there is error in these maps, we're just obligated to simulate the past PLSS as best we can. Ken ... View more

Jim,    You've really been a big help to me in past threads. I'm really grateful for your response.    To clarify: my team is attempting to get the PLSS lines used in 1910, into a geo referenced tif, so that we may geo reference some other old maps to the old PLSS tif. For these 1910 map tifs, we're not using map features for the 4 link points, rather we are using the lat and long coordinates found in each of the 4 corners of the maps. We are simply just entering the lat and long coords, printed on the map, into the link x y input box. I get a 1,000' discrepancy between the position of the old map tif section corners and new PLSS corners. I expected discrepancy, but does this seem reasonable? I wanted to use the nad27 datum, but I don't know how to get the arcmap's layer properties into lat and long units, using the NAD27 datum. Based on this additional info, what would you try?   You gave me a good idea: my team is hosting me to the San Diego conference in two weeks. Priority should be to track Melita down, if she is there, and show her a sample of my data. I'd like to meet you, also. Are you going to make the conference? If so, I owe you lunch. Ken ... View more

Our data is in a personal geo dbase and we're trying to replicate to another personal geo dbase, located in the same file. I was not clear, until your reply, that I definitely needed arcSDE to create the parent geo dbase. My question, to the group, is is there a work around, to set up a simple multi editor scenario without arcSDE? Also, is create replica the best tool to use, if, somehow, we can get an SDE parent geo dbase created? Thanks for your prompt response, Ken ... View more

Robert,   Thank you so much for your thorough and helpful answer. Both of us had read up on it, but missed the point that validation happens in a diff, more manual, way than for coded value domains, where we get a pick list of allowable choices. With your help, we are pleased to be able to set the option for automatic validation, via: Customize - arc map options - check the box for "auto validate records when editing" and then get the warning message when we enter a number outside of our range.  Only in an editing session. We are really grateful for your help. Ken ... View more

Neil,   Since the start of the discussion we have since changed our procedure to georeffing into utm, meters and so, now, our pixel size = 0.65 Hopefully, those are meters. I don't think we should create a new file; which is what, I'm assuming, you're refering to as creating a new file via changing the existing rectified image's projection. We simply just changed the projection of the arcmap we are using to geo reff to utm, meters.  Thank you for suggesting how we could optimize the rectify the settings, we attempted to use the esri help, with no success. Thank you. Ken ... View more

Neil, 1,1 is the pixel size according the the properties report via ArcCatalog. Based on your earlier comment, about pixel size, that must be 1 x 1 meter. Thanks for explaining the scale. I looked, unscuccessfully, for scale in the properties.  We know to be zoomed way in when setting link points.   We'd really like to get your opinion on the Update Geo referencing vs Rectify options.  If you prefer to use the Rectify option what adjustments to you make the Rectify menu options dialog. We've researched this and don't know how to optimize the settings. If you don't use rectify then let us know, so we can be assured we are doing the right thing by just doing update geo referencing for our scores of tiffs. Attached is a capture of the Rectify dialog box. I couldn't figure out how to insert it directly into this window. Our gratitude for your continued help to us, Ken and the 3 other DWR mapping section analysts ... View more

Neil,    Following up on your last thread, I want to evaluate the pixel size of the imagery, we are using, and I'm not I'm interpreting the imagery's properties. Cell Size = 1,1    Is this 1 meter by 1 meter?  So, if my RMS is off 3 m that would not be considered serious error? Pixel type= unsigned integer pixel depth= 8 bit By scale of the reference data, do you mean how far we are zoomed into the imagery when we do a link? Ken ... View more

Neil,   Your 2 comments were really helpful, as we realized Melita's calcs were for a point on the equator, but didn't know how to convert to our latitude of 39 degrees. That makes a big diff, since our error drops from 32' to 9', which is what I expect from setting 8 decent link points. Plus, we just did another geo reference, using the native projection, as you suggested, UTM, zone 11, rather the WGS84, I used in my example and got an RMS of 15' with just 5 links. In that case, our individual link points are listed in the link table in utm coordinates, meters, and we have a Total RMS error = 8  Is a conversion required, or should we interpret this error as 8 meters?   We'd like to get the group's opinion about what the lowest RMS error / minimum link points should be for old tiffs? I'm really grateful to you and Melita for assisting our team of 4 gis people. We have scores of old tiffs to geo reference for a water rights mapping project. Ken ... View more

Melita,    It made my whole day to see your math in the last note! I was able to use it to arrive at my earlier calc, done with diff math, of 9.8m.  It re assures, and makes sense, to me to get a realistic result, like 9.8 meters of error,  for the tiff I geo referenced, instead of the really small number I misinterpreted.  Thanks so much, Melita, from me and the other 3 gis analysts on our team, for helping us understand this process better. We have hundreds of historical tiffs to geo reference in the next year. For the other folks following this discussion: let me try to sum up the process of interpreting the RMS error reported in  the link table, using Melita's math : determine that the figures under the X & Y map columns are lat and long coords  The RMS error is then in decimal degrees. Our RMS error * 3600 seconds = converts the decimal degrees figure to seconds.  The RMS seconds * 30.922 = the error, in meters  Of course, this is the error at the equator and should be converted to your lat, if you need the extra accuracy. I'll post a follow up thread, showing my RMS error corrected for my lat, after I figure out how to convert it. I think the definition of the RMS error: the error between the coords of the link points set on the tiff and the coords of the  link points set at the same position on the NAIP imagery. I hope someone will feel free to correct my definition, if I'm in error. Ken ... View more