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Projections question at continental and local levels

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#1
Daniel25

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I know projections and coordinate systems are discussed at great length on every forum, but I haven't been able to find a good answer on the following:

We do a lot of mapping on a continental US scale. For this, USGS uses Albers Equal Area, so if it's good enough for them, it's good enough for us. So that makes our maps NAD 83 / Albers Equal Area.

Here's my question - I'm afraid that at some point, someone will want to work in a localized projection - say State Plane, so my question is if I want to make a new shapefile now or reproject something from NAD 27 etc, do I set the coordinate system of the shapefile to a geographic NAD 83 or do I set it to what my map is NAD 83 / Albers? Same goes with georeferencing a paper map to the Albers map; When I finish, and update georeferencing, should I just set it to a geographic or projected coordinate system?

It seems like all the data lines up (in my map) either way, but I just want to make sure my data will be useable by everyone, no matter what projection they use down the line.

Thanks for any insight.

#2
SaultDon

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Truth being, in projections, there is no right projection. They all cause some level of distortion.

Being clear to the end-user as to what the data is projected in is the best bet to make sure your data is usable by everyone. If this isn't communicated clearly, the user is left to Identify the Unknown Coordinate System.

That person should then understand Geographic Transformations and how to pick the right one.
Choosing an Appropriate Transformation

ESRI Help Topics:
What Are Map Projections?
About Projected Coordinate Systems
About Geographic Coordinate Systems

Those help topic links will have other links at the bottom of each help section to help you browse more similar topics.
Choosing a projection can often be decided base on practicality (who uses the data and for what; analytical or cartographic?).

Book:
Recommended Reading - Lining Up Data in ArcGIS: A Guide to Map Projections ISBN: 9781589482494


As for your other question: "if I want to make a new shapefile now or reproject something from NAD 27 etc, do I set the coordinate system of the shapefile to a geographic NAD 83 or do I set it to what my map is NAD 83 / Albers?"

If you only want your data to be drawn using a different projection that what it is currently projected in, ArcGIS does this on-the-fly by changing the dataframe (right click data frame > properties > coordinate system tab) coordinate system. This forces the data in the map to draw using that projection without actually changing the projection (transformations can be applied). If you like what you see, the data can then be exported (right click layer in Table of Contents > Data > Export Data) using the Projection of the Data Frame (not the source data!).

This is helpful if you want to see what the data will look like (transformation can also be applied) in a different projection, or want to get some measurements (using geodesic distance, for example).

Blogs: on buffers and projections
Using the Buffer Tool in ArcToolbox (and comparing with the Buffer Wizard in ArcMap)
Using the Buffer Wizard in ArcMap

Maybe Melita K. (ESRI) will take notice of this forum and elaborate for you. She is very knowledgeable on the subject and any projection questions I have I usually search the ESRI forums for her responses included.

#3
Melita Kennedy

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I know projections and coordinate systems are discussed at great length on every forum, but I haven't been able to find a good answer on the following:

We do a lot of mapping on a continental US scale. For this, USGS uses Albers Equal Area, so if it's good enough for them, it's good enough for us. So that makes our maps NAD 83 / Albers Equal Area.

Here's my question - I'm afraid that at some point, someone will want to work in a localized projection - say State Plane, so my question is if I want to make a new shapefile now or reproject something from NAD 27 etc, do I set the coordinate system of the shapefile to a geographic NAD 83 or do I set it to what my map is NAD 83 / Albers? Same goes with georeferencing a paper map to the Albers map; When I finish, and update georeferencing, should I just set it to a geographic or projected coordinate system?

It seems like all the data lines up (in my map) either way, but I just want to make sure my data will be useable by everyone, no matter what projection they use down the line.

Thanks for any insight.


If you're only working in the (contiguous) US, it simplifies things a bit. ArcMap loads the NADCON-based transformation, NAD_1927_To_NAD_1983_NADCON, automatically. This is used for the lower 48 states. Alaska and Hawai'i have different transformations between NAD27 and NAD83. Currently, ArcMap supports one transformation per pair of GCS (datums) rather than using a layer-based list of transformations.

In general, I would recommend setting the coordinate system of new data to your standard. That keeps everything simpler. If someone has data that's in NAD27 or a State Plane or UTM system (based on NAD27 or NAD83), that will all just work as everything will be projected on-the-fly (in memory) to the data frame's coordinate system. If someone wanted to edit in a local coordinate system, like a UTM zone, they should only edit in that area. For instance, if I need to edit some Ohio data (the Buckeye state!), and sets the data frame to NAD83 State Plane Ohio South, they should not try to edit data in New York or California or Texas in that coordinate system.

With rasters, if you need to georeference, I personally think it's better to georeference it to its current coordinate system, if known. If not known, then do what you have to.

Hope this helps!
Melita




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