shrug-l: Lat-Long

Thu Dec 22 12:07:43 EST 2005

Thank you Janis.  This is the point I was making.  As I'd mentioned
previously, it is the "on the fly projection" in ArcGIS that makes them to
overlay, but as you pointed out, when you use a product that doesn't have "on
the fly projection" you see that the geometry didn't change.  Only the
projection definition changed.

  _____  

From: Paulsen, Janis 
Sent: Thursday, December 22, 2005 12:04 PM
To: Hughes, Ronald; Thomas, Jim
Cc: shrug-l at lists.dep.state.fl.us
Subject: RE: shrug-l: Lat-Long

I thought I'd jump in here.  I just ran a little experiment.  I brought in
two differently projected shapefiles into ArcMap 9.0. They were reprojected
on the fly in ArcMap. In ArcMap, both shapefiles overlaid each other
correctly.  I set the dataframe coordinate system to GCS North American NAD
83 HARN.  I did a data export on both shapefiles selecting the "use same
coordinate system as data frame" option.  I then brought both shapefiles into
ArcView 3.x.  The both loaded into the apr, but did not overlay eachother.
This tells me that AcrView allowed them both to be brought in because they
were assigned the same coordinate system, but they were not actually
reprojected geometrically, since they did not draw in the same place in
space.  Interesting.
Janis

-----Original Message-----
From: shrug-l-bounces at lists.dep.state.fl.us
[mailto:shrug-l-bounces at lists.dep.state.fl.us] On Behalf Of Hughes, Ronald
Sent: Thursday, December 22, 2005 11:47 AM
To: Thomas, Jim
Cc: shrug-l at lists.dep.state.fl.us
Subject: RE: shrug-l: Lat-Long

Sorry Thomas, but it doesn't re-project.  That method you used simply
re-defines the projection unless they've changed it in Version 9x (which is
highly doubtful).  If you want to change the geometry, you'll need to use
ArcToolbox.  It's simply isn't good practice to assign incorrect projection
files to spatial data.

I hope that clears things up.

  _____  

From: Thomas, Jim [mailto:jim_thomas at golder.com] 
Sent: Thursday, December 22, 2005 11:33 AM
To: Hughes, Ronald
Cc: shrug-l at lists.dep.state.fl.us
Subject: RE: shrug-l: Lat-Long

It's not simply re-defining.  It's re-projecting.  

When you use the 'Export Data...' tool from ArcMap you can select the export
coordinate system (see attached jpg).  It will re-project the data to what
you define.

The Shruggers can use whatever method works best for them.

  _____  

From: Hughes, Ronald [mailto:Ronald.Hughes at dep.state.fl.us] 
Sent: Thursday, December 22, 2005 11:23 AM
To: Thomas, Jim; shrug-l at lists.dep.state.fl.us
Subject: RE: shrug-l: Lat-Long
Actually Thomas, the problem with that is that the NEW shapefile will still
be in STATEPLANE projection.  In other words, the geometry or "physical
condition" remains as STATEPLANE.  All you've done is re-define the
projection file.  Defining a projection and actually RE-PROJECTING a
shapefile are two separate things.  Re-projecting a shapefile involves
changing its geometry and topology while re-defining the projection only
changes a text file.  You never want to assign an incorrect projection file
to a shapefile because it tends to lead to trouble if you send that file to
somebody else and then they try to re-project that shapefile using incorrect
projection information.  

The reason that it appears to work for you is because of the ArcGIS
"projection on the fly" capability.  If you were to use a GIS product that
doesn't project "on-the-fly" you would see that re-defining the projection
file hasn't changed the geometry of the shapefile: it's still in STATEPLANE.

As far as datum transformation is concerned, when switching between certain
coordinate systems, you may wind up using different datum and need to
transform from one datum to the next.  For example when I was using ArcGIS
version 8x and projected from STATEPLANE NAD83 datum to Geographic North
American 1983 HARN datum I would get the following error message:

I would then have to set the transformation from NAD_1983 to HARN_Florida:

Many people think that redefining the projection of a shapefile is the same
and projecting a shapefile but it is not.  I hope that I've contributed to
clearing this up.

Good luck,
Ron

  _____  

From: Thomas, Jim [mailto:jim_thomas at golder.com] 
Sent: Thursday, December 22, 2005 10:52 AM
To: Hughes, Ronald; shrug-l at lists.dep.state.fl.us
Subject: RE: shrug-l: Lat-Long

The steps I listed will work too.  You can export a NEW shapefile from ArcMap
in the spatial reference of the data frame (which can be different than the
original shapefile).  As long as everything is defined correctly, you'll have
no problem.  I do it all the time.

As far as the datum transformation list... that's a good question. I always
wondered that.

  _____  

From: shrug-l-bounces at lists.dep.state.fl.us
[mailto:shrug-l-bounces at lists.dep.state.fl.us] On Behalf Of Hughes, Ronald
Sent: Thursday, December 22, 2005 10:32 AM
To: shrug-l at lists.dep.state.fl.us
Subject: RE: shrug-l: Lat-Long
Actually, you need to use ArcToolbox to reproject the shapefile.  All the
steps mention below do is re-define the projection file.  It would be
analogous to assigning a new mailing address to you home without physically
moving it from its current location.  In other words, if your house sits at
800 Main Street and you change the address to 21 Jump Street, the house still
remains at 800 Main Street.  

The Present version of ArcToolbox that I have (ArcGIS 9.1) is different from
that what I'd previously had but it appears to work the same way.

Open up ArcToolbox and select Data Management Tools --> Projections and
Transformations --> Project. 

Click on Project and a menu should come up that asks for your imput feature
and output (in ArcGIS 9.1 it automatically assigns a 'P' at the end of the
name of the input and places it in the output.  Select the projection that
you want for the output by clicking on the button to the right of the input
window.  That will bring up a menu called "Spatial Reference Properties".

1.) Click the button labeled "Select".

2.) Click the Geographic Coordinate Systems folder

3.) Click the North America folder and select the coordinate system that you
want (probably North American 1983 HARN.prj).

4.) Click the "Add" button and select "OK".

If you have any other questions just ask.  

Good luck!

-----Original Message-----
From: shrug-l-bounces at lists.dep.state.fl.us
[mailto:shrug-l-bounces at lists.dep.state.fl.us] On Behalf Of Thomas, Jim
Sent: Thursday, December 22, 2005 9:34 AM
To: tdarsey at hsa.cc; shrug-l at lists.dep.state.fl.us
Subject: RE: shrug-l: Lat-Long

1. Define your shapefile's spatial reference in ArcCatalog
2. Load the shapefile into ArcMap
3. Change the data frame spatial reference to Lat/Long
4. Export the new shapefile using the data frame's spatial reference (during
export)
5. That's it.

  _____  

From: shrug-l-bounces at lists.dep.state.fl.us
[mailto:shrug-l-bounces at lists.dep.state.fl.us] On Behalf Of Tony Darsey
Sent: Thursday, December 22, 2005 9:30 AM
To: shrug-l at lists.dep.state.fl.us
Subject: shrug-l: Lat-Long
Can anyone tell me how to convert a shape file from State Plane to Lat/Long?
Thanks, 
Tony Darsey 
HSA Consulting Group, Inc. 
1311 Executive Center Drive, Suite 255 
Tallahassee, FL 32301 
Phone (850) 309-7510 
Fax     (850) 309-7510 
Cell     (850) 508-6477 
email   tdarsey at hsa.cc 

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