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Archive for the 'coordinate conversion' Category

Earth Survey Plugin: NGS Benchmarks, PLSS Data, USGS Quad Data And More

A while back, I posted about a free web app from Metzger and Willard that shows National Geodetic Survey control points (benchmarks) near a specific area, and lets you view data for those landmarks. I’ve just noticed that they’ve created a newer web app called the Earth Survey Plugin, running in a Google Earth browser plugin that not only has the same capability, but also adds a bunch of additional features:

  • An NGS Survey Marker capability that works very similarly to the previous app, but now offers the ability to export the data into a static KML file
  • A PLSS point geocoder function that either gives you the section data for the point in the middle of the display:


… or lets you enter the PLSS parameters, and find the center point associate with them:


These can also be saved as a KML file.

  • A click-to-geocode function:


Plus, a set of overlays:

  • PLSS sections, including quadrants and subquadrants:


  • Principal meridians:


  • USGS topo quad index; orange dots for 1:24K, purple for 1:100K, cyan for 1:250K. Clicking on a dot brings up a pop-up balloon with the name of a quad, and a direct link to the GeoPDF for that quad at the USGS store. Note that GeoPDF quads are not currently available for quads in US National Forests, and that at this time, some states (e.g. AZ, CA) don’t have full topographic information on their GeoPDF quads.


  • Actual USGS topo map views, with the scale depending on your zoom level.

National Geodetic Survey Online Toolkit

The National Geodetic Survey has a set of links online conversion utilities for performing some basic geodetic conversions, including:

  • High-accuracy state plane coordinate system (SPCS) conversions
  • UTM/USNG/geographic coordinate conversions
  • Magnetic declination
  • Surface gravity prediction
  • Azimuth/distance to a second position (or second position from azimuth/distance data)
  • Position from a dual-frequency GPS data file

Plus many more. Some of these are also available as PC programs, but these tend to be old-school DOS-based conversion utilities, not always the most user friendly.

View NGS Benchmarks, UTM Zones, PLSS Meridians And More In A Web Interface

The Surveying.Org website plots a number of useful survey-related data features in a Google Maps interface; select one or multiple data features to display with a checkbox.

National Geodetic Survey Benchmark locations


Takes a few seconds for them to pop up. The different symbols correspond to various classes of accuracy for the benchmarks, both horizontal and vertical. Click on a benchmark icon …


… and get at popup with the name/designation of the benchmark. Clicking on the datasheet link brings up a full datasheet with coordinates, quality information, and more.


NGS Vertcon info



State Plane Coordinate System (SPCS) Zones


A few of the icons/popups are slightly misplotted, like the second one in the lower left, but they’re close enough to their respective SPCS zones to let you figure it out.


UTM Zones


This is the only dataset that’s good for outside the US; UTM zones are displayed for the entire world.


Meridian locations for Public Land Survey System (PLSS) designations (Township/Range/Section)


Also slightly misplotted in a few cases, as with the Navajo Meridian above, but close enough to figure out.


Area/length measurements


Finally, you can put the web app into either line length or area mode, and then click on the map to define vertices for a measurement. Above, an area is defined …


… and the area given in various units. Click and drag on a vertex to move it; Ctrl-click on a vertex to delete it. Doesn’t seem to be a way to add a vertex to a line segment, though.

Lookup EPSG Coordinate System Code From PRJ/WKT File

PRJ files are often included with GIS vector or raster data, and define the coordinate system associated with the data using the Well-Known Text (WKT) format. The Prj2EPSG utility site lets you upload or paste PRJ file data, and looks up the standard EPSG code designation associated with it (or gives you a list of what it thinks are the best matches). Utility seems to work well with many standard coordinate systems:


It’s not perfect, though. I threw an oddball Lambert Conformal Conic projection at it, from a USGS GeoPDF of a Utah 1:24K topo map, and it was a bit stumped:


Estonia and India ;)? To be fair, this particular coordinate system gives a number of other programs heartburn as well, and it was the only outright failure in 10 sample files; the other 9 came up with one candidate answer, and it was always the correct one.

Handy Set Of Online Calculators

APSalin, a company specializing in “Software development and data management for radio and television broadcast engineering”, offers a free set of handy online calculators and reference materials:

  • Converters to/from geographic/geodetic  –  Mercator  –  UTM – Cartesian coordinates
  • NAD27 to NAD 83 converter (latitude/longitude only)
  • Latitude/longitude DMS to decimal & vice versa
  • Great circle distance/bearing between two points
  • Destination (enter position-bearing-distance, get coordinates of destination)
  • Reference tables showing what distances on map represent for various map scales, and what various distances in real life correspond to on a map at a specific scale. Tables are given for both metric and imperial units; if you ever need an argument for doing everything in metric, this is a good place to start.
  • Reference tables for DEM resolutions in arc-seconds converted to meters and feet
  • Ellipsoid parameters for various datums

Compare Geographic Boundaries With Move Outlines

I can remember as a kid (way too many years ago) being impressed with a map of the lower 48 United States that had the outline of Alaska superimposed on top of it. The Alaska outline virtually covered the entire map, and there was a comment to the effect that,”Alaska is almost as large as the lower 48 states”. It wasn’t until years later that I realized the creator of that map had just traced an outline of Alaska off of a Mercator projection map and laid it on top of the US map without compensating for the change in scale.

If only the map maker had had access to  Mapfrappe’s Move Outlines site, he might not have made that mistake. Draw an outline of a geographic area in one Google Maps window:


And have the outline be superimposed on top of another Google Maps window, scaled correctly to compensate for changes in the Mercator scale at different latitudes:


Alaska’s still pretty dang big, but this shows it at its true scale, roughly one-third the size of the lower 48.

Another classic example of this is Greenland, which looks humungous on a standard Mercator projection:


In true area, though, it’s roughly the same size as Mexico; big, but not gargantuan:


The site has some pre-drawn comparisons, like the Great Lakes against the Black Sea:


Multiple Coordinate Systems In Google Maps, Reverse Geocoding, And More With The Worldwide Coordinate Converter

Clement Ronzon emails about his new website, The Worldwide Coordinate Converter (TWCC for short). Drag the globe-shaped icon to the desired location in a Google Maps interface, and get a pop-up balloon with the geographic coordinates for that spot, elevation in meters, and the nearest reverse-geocoded address:


At right is a two-part coordinates box, with latitude/longitude/WGS84 always in the top part, and a user-selectable coordinate system at the bottom:


You can also enter coordinates into the appropriate boxes in either the top or bottom section, click Convert, and have them converted to the other coordinate system automatically (and plotted on the map).

Look Up EPSG Codes From Well-Known-Text (WKT) / prj Files With Prj2EPSG

OpenGeo announces Prj2EPSG, a free online service that can look up the EPSG (European Petroleum Study Group) code for a coordinate system based on:

  • A WKT (Well-Known Text) description. e,g, copying and pasting WKT text into the box:


yields the EPSG code 4326.

  • A .prj file, often found with shapefiles or other geographic data files to define the coordinate system; select the prj file to upload, and get back the original prj data in the box and the EPSG code:


  • Type in keywords and get back a list of possible choices:


  • Or just type in the EPSG code, and get back a data page with the coordinate system defined, and the WKT listed.

Click on any of the code links in the search results to get a full data page:


It’s kind of picky about how you describe the data, and not necessarily in a consistent fashion. For example, enter “NAD 27” and you’ll get no results, but enter “NAD27” and you’ll get results; enter “WGS84” and no results will come up, while “WGS 84” yields lots of results. Only the top 20 results display, and that’s not always enough to find what you’re looking for; there’s no option to move on to another page of results, or change the total number of results displayed. Plus, partial matches aren’t allowed; entering “SWEREF” or “SWEREF 99” yields no results, you have to specify “SWEREF99”. So for now it’s a useful adjunct to, but not yet a replacement for, the Spatial Reference website that also allows searching of the EPSG code database with keywords, and which is more flexible in matching terms (e.g. it will find just “SWEREF”). But unlike Spatial Reference, Prj2EPSG has a web service interface that allows basic search queries.

Via SlashGeo.