Brave Adventurers ONLY: Seriously. How to find an EPSG 401
Mar 9, 2019 21:25:37 GMT -5
nevadaballin, jaywhyenecks, and 1 more like this
Post by chadgolf on Mar 9, 2019 21:25:37 GMT -5
IF YOU AREN'T COMFORTABLE WITH HANDLING TEN OR MORE COMPLEX GEOSPATIAL NUMBERS AT ONCE, JUST MAKE A THREAD ASKING FOR HELP AND THE COMMUNITY WILL HAVE YOUR BACK SOON.
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We're reaching the point where I can't keep up with the requests for EPSG codes, so I'm going to walk those brave enough to help others through the process.
I'd love to find a map where you can click and find all of the possible projections for that point, but I haven't found it, so here's the rough process.
But first we need to describe the types of EPSG you may find.
Now that we have some idea of what we're looking for, it's time to dig for information.
You may find a few EPSG codes inside the XML metadata. These are just in a format that the tool doesn't recognize, Google these until you find one that represents the specific area you're looking for. You're done! Nice
You may find a complex list of numbers like:
The names and formats of the numbers will vary, but after a while you'll grow to recognize which ones you need to match. Keep these numbers visible and move onto the next step:
If you can't find the numbers (or have the numbers) you now need to figure out what the possibilities are and which ones match. In this case, we know the course is in Knoxville, TN, USA. So we will use spatialreference.org/ and search for "Tennessee". We lucked out! There's only a couple. If there were more like California, Michigan, or Wisconsin, we'd have to go through each one.
Starting at the top of search results, open each EPSG code link and click on "Human-Readable-OGC WKT". You'll now see even more numbers! Go through and make sure that a few key numbers match. If only a few do, you'll likely have to go back and find a different frame with a different unit (especially if all match expect False easting and False northing).
In this example, I clicked on EPSG:2204 for 'NAD27 / Tennessee'. Unfortunately, we've already found an issue because NAD27 is not equivalent to our 'GCS NAD 1983 2011'. We're looking for NAD83. Go back to the list.
Next is EPSG 2274: NAD83 / Tennessee (ftUS). This looks better. Let's click on Human Readable OGC WKT.
Let's go down the OGC WKT and match important numbers:
Since False Easting and False Northing, and the other numbers ALL match to many digits, I feel confident that our Magic EPSG number IS EPSG 2274!
A few things to keep in mind, there are TWO different values for feet. Using the wrong one will lead to a bunch of small but terrible results.
US Foot is US Surveyor Foot is 0.3048006096012192
Foot or International Foot is 0.3048
And again, if most numbers match but False Easting and False Northing don't match, check your units.
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We're reaching the point where I can't keep up with the requests for EPSG codes, so I'm going to walk those brave enough to help others through the process.
I'd love to find a map where you can click and find all of the possible projections for that point, but I haven't found it, so here's the rough process.
- You're here because the automated process for finding the metadata has failed. That's tough and means we gotta get our hands dirty to find out the right EPSG code. Without this, OSM and Sat preview won't line up correctly.
- We don't know exactly what files the user has acquired, so at least ask for the console text and it will show some example names. These files are usually easy to find on the National Map or other lidar database.
- You'll probably want the course name and city/state as well. I think I ask for those in the other stickied post.
- Now that you have this information, we're going to look for what the possible EPSG could be. For example, we'll use this thread as an example:
tgctours.proboards.com/thread/18301/epsg-help-cherokee-country-club
But first we need to describe the types of EPSG you may find.
- Ellipsoidal EPSG - These describe how circular or not the Earth is, not what we're looking for.
- Altitude - These describe the where sea level is, we are also not looking for this one.
- UTM - This one is easy. It's a huge chunk of either the Northern or Southern hemisphere and they all have an easy number like 11 (or 11N). If you find this, just Google for EPSG UTM ZONE and make sure your city/state/course is in the huge area.
- Other Local Systems - You might find easy ones like United Kingom (27700) but pray you don't need to deal with Alaska. All countries and US States are unique and you'll need to find out the specific ones AND match the units (feet / meter). Even if the "official" state documentation says they use feet, there may be International Foot, US Survey Foot, or Meter unit versions of all local systems.
Now that we have some idea of what we're looking for, it's time to dig for information.
You may find a few EPSG codes inside the XML metadata. These are just in a format that the tool doesn't recognize, Google these until you find one that represents the specific area you're looking for. You're done! Nice
You may find a complex list of numbers like:
<gridsys>
<gridsysn>State Plane Coordinate System 1983</gridsysn>
<spcs>
<spcszone>4100</spcszone>
<lambertc>
<stdparll>35.25</stdparll>
<stdparll>36.4166666666667</stdparll>
<longcm>-86</longcm>
<latprjo>34.3333333333333</latprjo>
<feast>1968500</feast>
<fnorth>0</fnorth>
</lambertc>
</spcs>
</gridsys>
<planci>
<plance>coordinate pair</plance>
<coordrep>
<absres>0.000328083333333333</absres>
<ordres>0.000328083333333333</ordres>
</coordrep>
<plandu>Foot_US</plandu>
</planci>
</planar>
<geodetic>
<horizdn>GCS NAD 1983 2011</horizdn>
<ellips>GRS 1980</ellips>
<semiaxis>6378137</semiaxis>
<denflat>298.2572221009999052990208776</denflat>
</geodetic>
</horizsys>The names and formats of the numbers will vary, but after a while you'll grow to recognize which ones you need to match. Keep these numbers visible and move onto the next step:
If you can't find the numbers (or have the numbers) you now need to figure out what the possibilities are and which ones match. In this case, we know the course is in Knoxville, TN, USA. So we will use spatialreference.org/ and search for "Tennessee". We lucked out! There's only a couple. If there were more like California, Michigan, or Wisconsin, we'd have to go through each one.
Starting at the top of search results, open each EPSG code link and click on "Human-Readable-OGC WKT". You'll now see even more numbers! Go through and make sure that a few key numbers match. If only a few do, you'll likely have to go back and find a different frame with a different unit (especially if all match expect False easting and False northing).
In this example, I clicked on EPSG:2204 for 'NAD27 / Tennessee'. Unfortunately, we've already found an issue because NAD27 is not equivalent to our 'GCS NAD 1983 2011'. We're looking for NAD83. Go back to the list.
Next is EPSG 2274: NAD83 / Tennessee (ftUS). This looks better. Let's click on Human Readable OGC WKT.
Let's go down the OGC WKT and match important numbers:
- NAD83 - Matches to GCS NAD 1983 2011
- UNIT["US survey foot",0.3048006096012192 - Matches to <plandu>Foot_US</plandu>
- PARAMETER["standard_parallel_1",36.41666666666666] - Matches to <stdparll>36.4166666666667</stdparll>
- PARAMETER["standard_parallel_2",35.25] - Matches to <stdparll>35.25</stdparll>
- PARAMETER["latitude_of_origin",34.33333333333334], - <latprjo>34.3333333333333</latprjo>
- PARAMETER["central_meridian",-86], - Matches to <longcm>-86</longcm>
- PARAMETER["false_easting",1968500], - Matches to <feast>1968500</feast>
- PARAMETER["false_northing",0], - Matches to <fnorth>0</fnorth>
Since False Easting and False Northing, and the other numbers ALL match to many digits, I feel confident that our Magic EPSG number IS EPSG 2274!
A few things to keep in mind, there are TWO different values for feet. Using the wrong one will lead to a bunch of small but terrible results.
US Foot is US Surveyor Foot is 0.3048006096012192
Foot or International Foot is 0.3048
And again, if most numbers match but False Easting and False Northing don't match, check your units.