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Why does part of a line not project the sam...   jon Syder Oct 01, 2008
Re: Why does part of a line not project the...   Melita Kennedy Oct 02, 2008
Re: Why does part of a line not project the...   jon Syder Oct 02, 2008
Re: Why does part of a line not project the...   Melita Kennedy Oct 02, 2008
Re: Why does part of a line not project the...   jon Syder Oct 02, 2008
Re: Why does part of a line not project the...   Melita Kennedy Oct 03, 2008
Re: Why does part of a line not project the...   jon Syder Oct 04, 2008
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Subject Why does part of a line not project the same as the full length line? 
Author jon Syder 
Date Oct 01, 2008 
Message I have a line from A to B, 272 km along a line of latitude (59°). This is a fishing area boundary south of Prince William Sound, Alaska. The line is between two points in a point file. When I trace part of the line in WGS_1984_PDC_Mercator and subsequently change the data frame projection to Alaska_Albers_Equal_Area_Conic, the traced line segments no longer align with the parent line. If I start at either end, the origin of the segment and parent coincide with the point file location but the “free” end of the segment is south of the parent line. If I trace a segment of the line in the center of the parent line, it moves “parallel” but south 2.3 km of the parent line when I change the data frame projection. If I trace the whole line length from point to point it aligns perfectly with the parent from end to end, in both data frame projections. If I snap a line from point to point rather than tracing it aligns perfectly with the original line from end to end in both data frame projections. If cut the end-to-end-perfectly-aligned-in-both-projections-traced-copy, leaving a segment in the middle of the parent line and change the data frame projection it moves in the same direction and magnitude that the traced segment does. Is there a way to have the segments and the master line stay in alignment?

The entire rest of the closed polylines, that these latitude lines form the southern boundary of, stays in perfect alignment in all data frame projections.
 
   
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Subject Re: Why does part of a line not project the same as the full length line? 
Author Melita Kennedy 
Date Oct 02, 2008 
Message Hi,

We don't densify features when we project them for performance reasons. A conic projection will have curved latitude lines, while they're straight in Mercator or "geographic" (pseudo-Plate Carree when displayed in ArcMap).

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Why does part of a line not project the same as the full length line? 
Author jon Syder 
Date Oct 02, 2008 
Message Well, I have multiple features that have the same origin in the south west corner with a latitude southern boundary of different lengths. In Alaska Albers the shorter boundaries drop below the longer boundaries and create a saw tooth effect along that boundary. It’s pretty hard to tell my users that “everything is ok because we don’t densify when we project” and that those boundaries are really in the same place. How come the longer line isn’t curved to match the shorter ones? Is there a fix even if it is slower? Thanks for your help. jon 
   
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Subject Re: Why does part of a line not project the same as the full length line? 
Author Melita Kennedy 
Date Oct 02, 2008 
Message Hi,

Currently, the only fix is to densify (add more vertices to) longer features.

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Why does part of a line not project the same as the full length line? 
Author jon Syder 
Date Oct 02, 2008 
Message Hi Melita,
Is there away to do this in an efficient manner. I don't see any mention in the help file. Putting vertices in manually does not seem to make sense and I'm sure there is a sensible number or spacing to get a reasonable curve. We have ArcView licenses. Thanks. jon 
   
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Subject Re: Why does part of a line not project the same as the full length line? 
Author Melita Kennedy 
Date Oct 03, 2008 
Message I know there isn't a tool yet. I just heard that we're thinking about emulating the densifyarc command (from ArcInfo workstation) as a Toolbox box for a future release. I'm quite happy about it.

It looks like Dan Patterson has posted a python-based program to densify polylines or polygons:

http://arcscripts.esri.com/details.asp?dbid=15573

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Why does part of a line not project the same as the full length line? 
Author jon Syder 
Date Oct 04, 2008 
Message Hi Melita,
After looking at your messages I finally deduced that it was possible to actively densify a line rather than it being a passively determined quality. What I was having difficulty understanding was why all the other lines in each feature projected satisfactorily whereas the lower line did not. Some searches later I found and set up Dan Patterson's Densify toolbox, which is based upon a python script also included in the zip file. Although it is unclear what the units are, or what is a sensible number, it does work very well. The short lines and the longer lines now align. Perhaps you could put a note into the help file on "densifying", it relation to projections, and incorporate Dan's toolbox in the next build. Thanks. jon