Datums Conversion FAQ
What should I do if I want to transition into NAD83?
- VCGI public coverages
- Digitized coverages, custom coverages and shapefiles
- Convert to NAD83 using Arc/Info PROJECT command
- Use ArcView PROJECTOR utility and TransNAD (3rd party shareware)
- GPS data
- Export original GPS data to new coverages in NAD83
What's a Datum?
-
A datum provides a reference for surveying and mapping the Earth.
- It is a mathematical model of the Earth that includes a set of precise
measurements of the Earth.
- In summary, a datum is the mathematical model that serves as the foundation for
spatial referencing of the Earth.
Why is some data in NAD27 and others in NAD83?
- Until recently, most contemporary maps of Vermont used the NAD27 as the
reference datum. This made it the logical option for direct digital conversion and
subsequent inclusion in the Vermont GIS database.
- With the arrival of the new Vermont Digital Orthophotography, expanded GPS
activity and new maps from the U.S. Geological Survey being prepared using
NAD83, it is increasingly necessary to convert existing NAD27 data to NAD83.
Is there a significant difference between NAD27 and NAD83?
- Significant difference exists between NAD27 and NAD83.
- The shift to NAD83 affects the location of all latitude/longitude values. Locations
of some points are shifted by as much as 160m. In Vermont, the difference
averages about 35m in magnitude.
What is a State Plane Coordinate System?
- The state plane coordinate system is a set of conformal map projections that cover
the entire United States.
- A conformal map projection preserves the property of shape; for a small area,
angles measured on the map equal angles measured in the field.
- The original state plane coordinate systems were developed using NAD27 during
the 1930's. With the development of NAD83, there have also been modifications
to state plane coordinate systems.
What do Vermont Statues have to say about this stuff?
-
Title 1 of Vermont Statutes Annotated, Chapter 17, sections 671-679, defines the
Vermont Coordinate System of 1927 (VCS27) and Vermont Coordinate
System of 1983 (VCS83) as the state plane coordinate systems for Vermont.
- VCS83 will be the sole system for projects commenced after January 1, 2000.
Why is there such a big difference between my NAD27 and NAD83 data?
-
Though the numeric values for the central meridian and origin are the same, they
are different locations because they refer to different datums. One very important
difference is the false easting parameter.
- The differential is an offset of 347,599.695m between the eastings the X value
-- of the two coordinate systems. One advantage of this large offset is that it is
easy to distinguish between VCS27 and VCS83--just examine the easting or X
coordinate.
How has the datum change impacted the Vermont Orthophoto base maps?
-
Vermont's orthophoto mapping program began producing image maps in 1974.
These maps are produced at a 1:5,000 scale and up until 1994 they were based on
NAD27.
- Beginning in 1994, the horizontal datum changed to NAD83. Thus all new
orthophotography that is available from the Vermont Orthophoto Mapping
Program will be based on NAD83.
Can I digitize using the NAD83 tics on the 1990 -1994 orthos for Chittenden, Windham and Bennington Counties?
-
Sheets produced between 1990 and 1994 covering these counties contain
approximate tic marks for the Vermont Coordinate System of 1983.
- These points are approximate and should NOT be used in an attempt to directly
register a NAD27 sheet in NAD83 coordinates for digitizing purposes. Sheets
should be digitized using NAD27 values and the projected to NAD83.
Has the datum change impact the ortho sheet numbering and area covered by each sheet?
-
Vermont orthophoto map sheets are 4,000 meters square (except for the 1:1,200
scale imagery over some urban areas).
- Corner values have numbers like 104,000m in the X or easting and 128,000m in
the Y or northing for VCS27. Each sheet was numbered using the first three
numbers of the X and Y coordinate values from the southwest corner of the
image.
- Since the change to NAD83 changes the Vermont Coordinate System, coordinate
numbers for the same location are different, but the new NAD83 map sheet is
designed to cover almost the same area as the NAD27 map sheet. For continuity,
the sheet numbering and naming of the NAD83 sheets remains unchanged.
- In summary the NAD83 map sheet has the same number as the NAD27 map sheet
and covers approximately the same area, but the southwest X coordinate or
easting no longer corresponds directly with the map sheet number.
What kind of base maps can I use for digitizing?
- a. Two different types of maps are generally used. These maps are the U.S.
Geological Survey 1:24,000 Topographic Maps and the Vermont 1:5,000 scale
Orthophoto Base Maps.
- For a great many years, each of these products will be available in a mix of
NAD27 and NAD83.
- Users of these maps MUST be cognizant of the datum employed for each map.
The datum and projection information is presented in the lower left corner of the
map sheet.
How do I handle projection/datum issues when digitizing?
-
Maps should be digitized in the projection/datum in which they are drawn.
- Once the data has been correctly digitized it can then be projected to the desired
projection.
- By following this general rule, the correct spatial representation of information
can be anticipated.
I want to create NAD83 data from NAD27 USGS Quad maps!
- If you are using a USGS quadrangle NAD27 paper map as your digitizing source,
digitize it normally and re- project it into NAD83 using the Arc/Info command
'PROJECT'.
- Use the VGIS coverage QUAD_27 to register your map sheet.
- It is NOT recommended that users try to digitize directly into NAD83 from a
NAD27 source map.
- VCGI can also provide users with the appropriate projection parameters file
(*.PRJ) when necessary.
I want to create NAD83 data from NAD27! orthophotos
- When using a NAD27 paper orthophoto as the digitizing source, insure that you
are using a copy of the VGIS coverage ORTHO_27 when registering to the
digitizer.
- Be sure to then use the Arc/Info 'PROJECT' command to re-project the finished
coverage into NAD83.
I want to create NAD83 data from NAD83 orthophotos!
- Do not use the VGIS coverage ORTHO. Although it is NAD83 datum, it is
simply a re-projection of the old NAD27 ORTHO coverage
- Use the VGIS coverage, ORTHO_83 when digitizing from an orthophoto that is
in NAD83 datum.
How do I convert my data using Arc/Info?
-
Use the Arc/Info PROJECT command to convert existing coverages (or
Shapefiles that have been converted to coverages via SHAPEARC).
- Usage: PROJECT <cover> <incover> <outcover> {projection_file}
- You can get {projection_file}s from VCGI. Just download a copy of the
NADTOOLS Data Product and refer to the *.PRJ files.
- Refer to Arc/Info help for additional information
How do I convert my data using ArcView? Option 1
- Load the ArcView "Projection Utility" extension (ArcView version 3.2 or higher)
- Open a view and choose "Projection Utility Wizard" from the file menu
- Step 1: Select the shapefiles to be projected
- Step 2: Click on "projected" as the coordinate system type, choosing "NAD_1927_ Vermont" as the name and "Meter" as units; then (IMPORTANT!!) click "Show Advanced Options;" click on the "datum" tab and choose "NAD_1927_To_1983_NADCON" as the geographic transformation
- Step 3: click on "projected" as the coordinate system type, choose "NAD_1983_ Vermont" as the name and "Meter" as units
- The remaining steps are self-explanatory
How do I convert my data using ArcView? Option 2
- Existing Coverages:
-
Convert existing NAD27 Arc/Info coverages to NAD83 using Arc/Info
PROJECT command. Requires ESRI's Arc/Info or DAK (Data
Automation Kit) products.
- Existing Shapefiles:
- Convert existing Shapefiles to Arc/Info (or DAK) coverages via
SHAPEARC command and then re- project to NAD83 using Arc/Info
PROJECT command. Requires ESRI's Arc/Info or DAK (Data
Automation Kit) products.
How can I access NAD83 data layers and conversion tools?
- Fill-out a VGIS Data Request Form (online or paper)
- Search and download files from VGIS Data Search Engine available in the VGIS Warehouse.
- Search and download NADTOOLS from VGIS Data Products Search Engine available in the VGIS Warehouse. Includes pcArc/Info SMLs for NAD conversion. An AML version will be available this spring.
- Corpscon
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