Browser tool · by All Things Spatial
The same point, two datums, forty meters apart.
Ten 1997 survey sheets sit on today’s imagery, and every monument on them is plotted twice: once in the datum the surveyor used, once in the one we use now. Georeferencing Explorer puts the Right-of-Way sheets from Wells, Maine, over modern imagery with the same survey monuments in the legacy NAD27 datum and in NAD83. The monuments never moved; the numbers that describe them did.
One WebMap, two datums
Where the data lives
The map is a public ArcGIS WebMap: ten tiled map services, one per survey sheet, draped over Esri World Imagery, plus two feature layers of survey markers. The sheets were scanned drawings in Maine West State Plane NAD27; georeferencing fitted each scan to control points the drawing and the imagery share, so the parcel lines on the 1997 sheet land on today’s lot lines.
A geodetic datum is the reference frame latitude and longitude are measured against. NAD27 was fit to North America from Meades Ranch, Kansas, on the Clarke 1866 ellipsoid; NAD83 is centered on the Earth’s mass on GRS 1980. Measured across the paired markers in this map, the shift is about 40 meters, almost due east.
- NOAA National Geodetic Survey: datums — the agency that defines NAD27 and NAD83.
- Geographic Datum Shift — the WebMap on ArcGIS Online that both builds load.
Watch it work
Watch the walkthrough
Sheet 1 over the imagery, the minimum scale, a NAD27 marker and its NAD83 pair in the Inspector, Layers, Legend, Bookmarks and the Basemap gallery, Scale & Extent, and the Help tab with the marker files.
How it works
What the map does
Sheets over imagery
The map opens on Sheet 1 at 1:1,128 with the drawing draped over the imagery and both marker sets on top. Each sheet has a minimum scale of about 1:36,000: zoom out past it and the sheets fade, leaving the markers on the imagery; zoom back in and the drawing returns.
One monument, two datums
Click a NAD27 marker and the Inspector in the sidebar shows its description, Northing and Easting in Maine West State Plane (US survey feet), longitude and latitude in decimal degrees, and the same in degrees, minutes and seconds. Click the NAD83 marker beside it for the same monument in the other frame, with its NAD83 state plane values in meters.
Layers, Legend, Bookmarks, Basemap
The widgets sit in the top-right corner of the map. Layers holds three groups: the survey markers, the ten Right-of-Way sheets, and the imagery. Legend keys the two marker layers. Bookmarks holds one view per sheet; sheet by sheet the survey walks the right-of-way northeast through Wells. The Basemap gallery swaps the imagery for Imagery Hybrid, Streets, Topographic or Navigation, and Home returns to the first sheet.
Scale & Extent
Reports the current scale, zoom level and WGS84 extent corners, each with a Copy button.
Help and sample data
The Help tab explains georeferencing, sets NAD27 against NAD83 in one table, and offers both marker files as CSV with the state plane definitions to import them: NAD_
Limits
What it does not do
- It does not transform coordinates. The two marker layers were published in their own datums; the map shows them side by side rather than converting one to the other.
- The 40 meter figure is local to this corner of Maine. The NAD27 to NAD83 shift varies across the contiguous US, from a few meters to over a hundred.
- The sheets are a 1997 record. Parcel lines, roads and buildings on today’s imagery may have changed since; the alignment is of the survey as drawn.
- It is a demonstration built by All Things Spatial on a public WebMap; the survey sheets and markers are the project record they came from, not an official datum product.
FAQ
Common questions
Why are there two markers for every monument?
Each monument is published once in NAD27 and once in NAD83. The gap between them is the datum shift; the ground has not moved.
Which marker is right?
Both, in their own frame. A coordinate only means something with its datum attached; modern GPS and ArcGIS use NAD83 (or WGS84), older plans and USGS topographic maps used NAD27.
Why does the drawing disappear when I zoom out?
The sheet tiles have a minimum scale of about 1:36,000. Past it the markers and imagery stay; zoom back in for the drawing.
Can I get the marker coordinates?
Yes — the Help tab offers both files as CSV, with the state plane definitions to import them.
Is the code available?
The app is a demonstration built with React on the ArcGIS Maps SDK for JavaScript over a public WebMap; the Calcite build runs the same WebMap on Esri’s design system. The patterns are the ones All Things Spatial builds for clients.
Build with us
Have scanned plans that need to land on today’s imagery?
Survey sheets, historic maps and engineering drawings, georeferenced and published as map services with the control you can inspect. All Things Spatial builds this on ArcGIS Pro and the ArcGIS Maps SDK for JavaScript.