Geoid 99

What’s the easiest and surest way can I utilize a SPCS with a legacy geoid? I would like to survey with NAD83(2011)-LA S zone with Geoid 99 for conducting flood elevation certificate surveys. We are supposed to set a benchmark in the field for contractors to set a new building at the correct minimum height but Geoids 12 and 18 are about 0.5’ higher than required, incurring additional costs for hitting so high a mark. I would like to avoid having to perform complicated calculations in the field to convert between geoids as much as possible.

Perhaps use THIS TOOL to adjust your points post process?

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It has to be RTK. Any post-processing requires multiple trips for what ought to be accomplished in one. I’m looking to establishing an elevation on a control point then transferring that known elevation to a benchmark set at a predetermined target elevation all within the same field trip.

How many points are you expecting to set? Are you using a base/rover or NTRIP?

I’m usually setting two control points and one benchmark. I normally complete this with SmartNet’s iMAX NTRIP so that the solutions refer to a CORS station. The NGS monuments in south Louisiana are not good for elevation due to our subsidence.

How about this site. It’s a tiff file of geoid99 up geoid2018

Adjusting a few points in the field with a laptop is too much?

Does Flow allow for loading user supplied geoids to a project?

This is the solution I’m hoping for.

With this solution, I might as well switch to using FieldGenius, which can directly utilize Geoid99.

I would prefer to use Emlid Flow for elevation certificates because I enjoy the simplicity and I can export the points in an excel sheet showing the location of the base station, which adds another check on my data. FieldGenius isn’t so simple and exports multiple, more complicated files but it works better for boundary and other work.

How do I utilize it in Emlid Flow though? I can get that and other files from the NGS website but I’m not sure how to add them to the list of vertical datums on Emlid Flow.

I think I found the solution in creating a custom coordinate system. It allows me to use legacy geoids but I have to manually define the other criteria of the coordinate system to match the definition of NAD83(2011) LA-S. I will test it out against a trusted monument tomorrow and see how it turns out.

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I’m not sure as I’m not as proficient in Emlid Flow as others are. There should be a way to import whatever geoid model you require I would imagine. I suppose Emlid uses some sort of .tiff file for their geoid models. If so, it would be a simple matter of defining the geoid for the projection needed. If not, this should be considered by Emlid.

Many field data programs allow this if the geoid surface is defined. If you can establish a geoid99 base point for your project area, it’s a simple matter of using that elevation for the project area if you’re using local RTK positioning. Look at NGS website, there’s a lot of information considering translation between vertical datums.

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Emlid uses .tiff files, but I don’t think you can upload your own. In this case, the OP could collect his few points, run them through the NGS page and be done. Seems trivial, but apparently won’t work for him.

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Friend, when you’re dealing with Louisiana heat/humidity, vegetation, and bugs, you want the fieldwork as efficient and brainless as possible to get you in and out as fast as possible and with as few calculations as possible.


I hope they let you have a car at least. :slight_smile:

It’s a one man op. I’m the one man lol


Give me a heads up ahead of time and then email me the coordinates and I’ll run them through the website and send them back to you! :grin:

It sounds like you could beforehand just pick a point in the survey area and run it through the website to determine the offset and just apply that general elevation offset when in the field?

Or maybe the custom coordinate system option you’re working on will work out.


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Depending on the size of the project site, we usually just use a mean of the 4corners of the site along with a center point to average the geoid height for the area. Using this simple method, you can see the overall slope of the ellipsoid or geoid for large (>500ac) areas.

As you probably already know this, you’ll see very little change in the geoid height on small projects. Also the error at the 95% confidence level for the NGS computation usually is 2-3cm.


I tried out the manually defined coordinate system (using the TIF that Emlid provides for Geoid99 in my zone) and carefully transcribed the defining parameters for NAD83(2011) LA-S zone. I tested it against trusted monumentation and against the Geoid99 model I use in FieldGenius and both checked out perfectly (vertically as well as horizontally). So apparently the way to use legacy geoids in Emlid Flow is to manually define your coordinate system to match the SPCS for horizontal and hold that legacy geoid for vertical.

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I’m not sure how much demand there is for using legacy geoids, but nice workaround.

I noticed on some recent OPUS files that NGS is tarting to work in xGeoid 19b in addition to Geoid 18