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A (very, very) deep dive into the computation of Elevation Scale Factors / Ellipsoidal Reduction for GRS80

In my previous blog/FAQ entry I noted that there will be a variety of ESF (Elevation Scale Factors) to be encountered for a given latitude and ellipsoid height.

In this arcane blog entry, I present the NGS Ellipsoid Reduction Method and a deep drive into the resulting differences for various flattening ratio values, compare results obtained with GRS80 vs. WGS84 ellipsoids, look at the scale of errors introduced by using the orthometric height instead of the ellipsoid height and provide links to additional resources.

Keep reading:

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I have been writing X-PAD FAQ’s every time I stumble over something interesting. Typically the items are very simple, but I think it is worth writing a bit about them to save others a bit of stumbling. Here is a quick synopsis:

All of the FAQ’s are in a single secure web [ folder ]

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What is the best Android tablet for X-PAD?

This entry started out as a link to the Samsung announcement of a new Tab Active tablet in October of 2022. But as usual, I have fallen off topic:

We used to sell data collectors with GNSS receivers. And we still do, however with our switch to X-PAD which runs on ‘basically any Android’ device, it is hard for us to sell data collectors because there are so many and the price points go from $80 to $650 for consumer tablets, all the way up to $3500 for a really nice Juniper Systems Allegro 3 or Mesa 3 data collector.

Which device should you purchase?

There are so many great solutions that it is really hard to make a suggestion. It is easier for me to tell you about the Android devices that I personally have: Pixel 4, Samsung 7, Samsung Tab Active 3. I plan to get a Pixel 7 Pro when they release in October and I am also going to upgrade to the Tab Active 4 when they release ‘any day now’.

Why don’t I have an Allegro 3, Mesa 3 or one of the other super-rugged Android devices? Well, I have gotten used to having the latest version of the Android OS on my phone and Samsung tablets. The Mesa 3 has Android 11 and the Allegro 3 has Android 7.1. The Bluetooth 5.x on the Pixel/Samsung seems to go 2 to 4 times further than the Bluetooth on these devices. The screen on the Tab Active has a lot finer detail. The new devices have full banded 5G modems, while slightly older devices have band limited 4G modems.

But the big difference is cost. The Tab Active 3 is $560 on Amazon with overnight delivery in Salt Lake City. That is 6.25 times less than a similar super rugged device!

Cell Phone vs. Tablet

I find that I use my cell phone 95% of the time. I use it for demos, I use it for support, I use it to test receivers and I use it for most survey related functions. If a customer calls me on my cell phone and I am talking on it, then I use my tablet to help with support. If I was doing construction layout or using the volume tools in X-PAD then I would use my tablet because it has a bigger, screen. But I don’t do construction layout or volume work.

The best data collector for me is the one that is always in my pocket.

Things to consider

Removable batteries. Time to charge: if device will charge from 20% to 80% in 20-minutes, then that is something. External charger for 2nd battery available?

Android OS Version.

Camera resolution and picture quality. Pixel and Samsung seem to have great image processing and I take a lot of pictures of cut stones. Will the Auto Focus quickly focus on a cut stone or brass cap?

Bluetooth version (5.x has much better range!)

Network Modem: 4G or 5G LTE? Verizon network compatibility? Network sensitivity. e-SIM?

Sufficient 4G/5G bands? Consider the difference between these two devices:

LTE: 2, 4, 5, 12, EU 1, 3, 7, 8, 20, 28, and AU 1, 3, 5, 7, 8, 28


#2: LTE: B1/2/3/4/5/7/8/12/13/14/17/18/19/20/25/26/28/29/30/38/39/40/41/42/48/66/71
5G Sub-610: Bands n1/2/3/5/7/8/12/20/25/28/30/38/40/41/48/66/71/77/78

The second device sure looks better!

Wi-Fi compatibility: Wi-Fi 6 (802.11 a/b/g/n/ac/ax, 2.4GHz + 5GHz, MIMO) or something else?

Rugged case availability. Mounting options for poles and ATV’s. Stylus compatibility.

Fast enough CPU speed? SD card slot for memory expansion (in case you want to put all the BLM field notes in PDF format on your device.)

Screen bright enough? Sunlight tolerance: most of these devices just turn off when they get hot. Screen resolution, not just size may be important for you.

What IP rating? (IP68 is probably best.) Which Gorilla(r) glass? (5 is good.)

I like a fingerprint reader for quick login. And since my devices are connected to my personal google accounts, I want them locked if I leave them around.

Operating temperature range? This may not be important for me because my personal operating range is so narrow.

Do you need a built in serial port?


Which device should you purchase? What ever makes you the happiest might be best.

I am really looking forward to a big new Pixel phone and will be excited to have a Tab Active 4 too!

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OPUS-Static: a deep dive into orthometric height estimated errors

A few years ago (perhaps 2019) the NGS changed the way that they computed the estimated orthometric peak-to-peak errors reported on an OPUS report.

Previously three items:

  • observed peak-to-peak ellipsoid value
  • estimated error for the geoid
  • and (perhaps) a county wide adder

combined to build an estimated error. The new method combines the observation duration, peak-to-peak ellipsoid observation, the estimated geoid error and the solution’s RMS error:

You would be correct to assume that nearly all occupations reprocessed using this new method will report higher estimated errors.

Here is an actual example, with the location obfuscated:

I made a simple spreadsheet to compute estimated errors using the new NGS method:

Here is a link to the Excel spreadsheet:

With a data table, we can look at the results of shorter and longer occupations:

It should come as no surprise that, all other things equal, even a 48-hour occupation will not match the 3cm result from 2019.

Before I suggest a solution to this higher orthometric estimated error ‘problem’, lets take a moment to compare some of the contributions other than time to the orthometric estimated error.

Baseline (distance) from CORS to your observation

When you process an OPUS observation soon after the observation completes, the nearest available CORS stations may be a long ways distant. In the example above, in 2022 after waiting 3-years to process, every possible CORS station will have posted data and the data will have been Quality Checked by the NGS. Bad stations will have been mapped out.

Closer CORS stations will typically have better matching ellipsoid elevations which in turn will reduce the reported orthometric error.

Good and Bad CORS

There are good CORS stations and bad CORS stations that suck. There are many reasons that make stations inferior:

  • nearby buildings or canopy
  • non chokering antennas
  • antennas without matching absolute calibrations (unlisted domes)
  • bad or insanely long antenna cables on the CORS stations
  • old CORS station firmware
  • poor mechanical mounts
  • … and many more …

OPUS (generally speaking) assumes that all CORS stations are perfect. Even if they are not. So if a station has a bad position, or has extreme multipath, then OPUS does not throw the station away. OPUS assumes the station is perfect and that all of the errors are the fault of your observation.

You should look at the short-term plots for the stations in a solution and exclude stations that suck.

Rapid vs. Precise vs. Final Orbits

While the ephemeris that are used in the OPUS solution don’t matter nearly as much as they used to, you may get a cleaner solution by waiting for precise orbits to become available, usually 15-days after the end of your observation.

Geoid accuracy

For the example above, the original solution was computed with GEOID 12B:

And the second solution used GEOID18:

The estimated error for GEOID18 is 1cm better than GEOID12B. But as you have seen, this does not translate into a smaller orthometric error estimate.

Solution: getting a lower error estimate

If your original occupation spanned longer than 4-hours, it will be possible to break it half and submit each half as a separate session into an OPUS-Project project.

While it would certainly be better to collect the raw observations on two separate days; or do morning/evening splits, with disparate physical setups; splitting the observation into two 2-hour or longer sessions will work.

Here are the results of the OPUS Project using all default settings:

with a 2.2 cm orthometric error estimate.

So, while the new OPUS-Static computation method for orthometric height estimated error may not get you where you want to be, OPUS-Projects probably will.

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SurvCE/SurvPC: FAQ Single or Multi-Point Localizations at Ground

Step-by-step instructions for setting up a ground system for a single point or multiple points linked below:

Single Point:

Two-Point Arbitrary Bearing:

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X-PAD: Stakeout an object, without a point

Suppose we would like to stakeout an object that does not have an associated point, like the midpoint of a line:

Stakeout the midpoint of a line.

The trick is to select the object’s point as coordinates:

Select from CAD

Detailed instructions with step-by-step screenshots are included in this FAQ:

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X-PAD: EntitlementID vs. EquipID/EquipSN

In early August 2022, GeoMax switched from an ‘EquipmenID, Serial Number’ license number pair to a single ‘Entitlement ID’. Existing EquipmentID and Serial Number pairs will continue to work forever, however if any option is added to a license or the X-Pert maintenance is extended, you will get a new entitlement ID.

The important thing to know is when you install X-PAD on a device with no activation, you can choose between entering an Entitlement ID or SN Pair:

Selecting a license type in X-PAD activation

And the secret to removing an existing license from your devices is to check it back into the cloud, called ‘Rehost license’.

This document describes the complete process, with detailed pictures:

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SurvCE/SurvPC Hawaii GEOID Files

Geoid 12B grid files for use with SurvCE/SurvPC are available here:

in .GSF, GSB and zipped versions of each.

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SurvCE ‘Clean Install’ alongside ‘Existing Install’

We feel that it is best when doing updates to SurvCE to do a ‘Clean Install’.

For example, if updating from SurvCE version 6.03 to 6.08, there have been several new options, new functions and major revisions to SurvCE. While it is possible to install 6.08 over the top of 6.03, we believe that it can be troublesome and lead to undefined option values.

There may be an advantage to keeping the old installation fully intact in case of problems, you can quickly revert back to the previous version.

This FAQ describes how I do a ‘Clean Install’: [ continue reading … ] preserving existing SurvCE activation.

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Carlson’s new RT4 Win10 Tablet Field Data Collector


I have had an RT4 for a few months and have used it with a number of GNSS devices and my Robotic Total Station. I wanted to take a few moments to write about my experiences and some random thoughts for those who might be considering upgrading.

Continue reading: [ RT4_Review_MES_003 ]…

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