# Geodesy, Ellipsoid, Geoid, etc

Global Mapper UserTrusted User
edited May 2015
Is there anyone out there that knows a lot about geodesy and related subjects?
I'm having trouble working through the relationships between the different aspects.
I have found quite a lot of info on the web, but not the answer I'm looking for...

I'll put it in the form of a hypothetical example, using GM12, SRTM30, UTM/WGS84:
Point A:
Sea surface = 0m
Depth of sea = -20m
Geoid = 0m
Point B (1000km away):
Sea surface = 0m
Depth of sea = -120m
Geoid = +40m
Problems and issues:
1. I assume that WGS84 is tied to the Ellipsoid (?)
2. Does SRTM30 reference to the ellipsoid or geoid?
3. Is simply generating bathy contours using WGS84 related to the ellipsoid; or is it referenced to the geoid or sea floor?
4. I want to know if the geoid makes any difference to the depth of water, sea level, etc where it differs from 0?
5. I am trying to create a map of Sundaland about 11.6k cal. yrs. ago. I have seen papers by Voris, et al., but I wonder whether the actual area of land above sea level (at the time) should be more or less than generated by contours from SRTM30.
6. In addition, I understand that -- depending upon the rotation speed of the Earth, axis, etc -- their could be additional changes in sea level over and above that resulting from the balance between the oceans and glaciers (inc Greenland, N & S Poles).

Help would be really appreciated.

• Global Mapper User Trusted User
edited May 2011
Chris,

“GM Mike” is a geodetic genius in my opinion, and can tell you what you need to know about that.

Regarding your project; Paleo-sea levels for any particular geologic time are estimated using radiometric dating in combination with various forms of proxy data, and much (hopefully !) scientific inference. The inherent range of error from these factors alone should be substantially greater than any correction for geodesy. Isostatic crustal adjustment from melting ice bodies would not be a factor in this equatorial area. Since “Sundaland” is the most tectonically active region on Earth, I would expect that endogenic processes would exert the greatest influence on the “sea level curve” there.

I believe current practice in paleo-geographic reconstruction would be to simply use unadjusted modern bathymetry in conjunction with whatever sea level you wish to portray. I see your issue though; a small sea level change makes a big difference in the extent of exposed land area in this region, but I think that this is “as good as it gets”. Personally, I would think about trying to illustrate the possible range of error graphically on your map too; perhaps with a transparent shaded area on either side of your reconstructed shoreline (like the error bar on a graph). That would be more scientifically proper.

Global Mapper is perfect for this sort of thing. I use it often for reconstructing relict landforms/landscapes etc., and for topographic analysis the same. It's also great for generating illustrations and diagrams for lectures and field trips.

Glad to help more if you wish.

Mark Pullen
Coeur du Deluge Chapter
Ice Age Floods Institute
mrpullen@yahoo.com

(p.s. - Why 11.6 ka ?)
edited May 2011
Actually Chris already asked me ("GM Mike") directly and I didn't know as I'm not a vertical datum expert yet, which is why he posted here for help from the community, such as the great response from Mark.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com
• Global Mapper User Trusted User
edited May 2011
(p.s. - Why 11.6 ka ?)[/QUOTE]
Thanks Mark for replying; its much appreciated -- especially as my previous posts got no response what so ever.
And yes, you may well be able to help further.
-- Mike -- thanks for adding that --
I'm a retired (due to ill health) Local Government Officer (in the UK). I'm trying to write an alterntive history novel with many maps; also have started an historical atlas of the Russian revolutions and civil war (1917-1923). I have interests in history and pre-history. I don't believe we know that there has never been previous civilisation(s) that current knowledge has not indentified (in terms of main stream knowledge).
The AH novel has it's roots in Plato's Atlantis (don't stop reading now...I'm NOT about to go on about aliens and spacecraft!).
Of all the material I have read over the years, a large inundated area (post LGM) seems the most likely place (even if it is not Plato's Atlantis). So, I am trying to see how the physical description of the city and plain fit somewhere in Sundaland.

The late Professor Arysio Santos (Brazilian) wrote a book: 'Atlantis: The Lost Continent Finally Found'. It is certainly interesting -- so long as you set aside his style and definitive claim -- that sets out some interesting arguements.

Like a lot of people sharing the same locational view, they see -120m and the rectangular Java Sea and , vola it fits. They forget about sea level rise and one look at the Voris maps shows that 11.6k is touch and go for the plain to fit.
I'm just an amateur at this, so I might ask some 'obvious' questions.
I've created contour maps by shading SRTM30, by generating contours (1m interval), made cross sections using the 3D path tool, etc.
Area: to see if it fits the description
Cross-sections: to see how 'level' the bathymetry is -- when is level considered to be level? (in general terms).
The late Prof. also believes that Krakatua blew its top (VE7?) and initiated the destruction. I'm not convinced, because Plato does not mention volcanos directly, but does mention earthquakes. The is a description of what must be floating pumice.
The Prof. also refers to a modelling study by Keyes and someone else claiming that Krakatua exploded 535AD; but thinks they got the date wrong and says it was probably about the time I'm talking about.

I have just read another book by Richard W Welch, 'Roots of Cataclysm', that introduces a Geopulsation Theory. Its not entirely clear to me whether he's 'invented' this theory or not. Essentially, he claims that in addition to sea level changes relating to glaciation (eustacy?); that the planet also goes through cycles of rotational spin up and spin down (my term) which results in the ocean being pushed towards the Equator during spin-up -- centrifugal forces.
If there is any reality to this claim, then the sea level at Sunda could be a lot different. An example he uses refers to a -400m drop and the North Pole and a corresponding increase on +200m at the Equator! I think this is at the height of the LGM, so it would obviously be less as the ice melts.

I'm guessing that main stream geologists, geodesists, geographers and other scientists would find outlandish. How do you feel about these things?

I accept your point about ranges -- it makes sense -- even if I always want more.

I would still like to know the answer to the relationships of levels, because I am frustrated at not being able to find the answer.

The question is also relevant to the novel research in another way -- the Caspian Sea.

Another source was: Eden in the East, Stephen Oppenheimer.
Indeed my interest in sea level stuff is World wide, including the Persian Gulf and North Atlantic.
My overall time frame of interest goes back 130k+ cal yrs.

So, Mark -- if you're still with me -- perhaps this is a discussion you might want to continue via direct e-mail -- if that can be arranged.
Any help would be most appreciated.

Chris
• Global Mapper User Trusted User
edited May 2011
Chris,

Although the liquid world ocean responds to changes in global ice volume as though it were separate from the solid body of the earth, in terms of geophysics, they are one (partly because the thickness of the ocean is so small compared to Earth’s radius). Variations in the geoid affect both the solid crust and the ocean; so from our relative perspective on Earth’s surface, it has no effect on sea level (and therefore, is not a correction to apply in paleo-bathymetry). The geoid is a surface of equal gravity values, and the oceans’ surface must mimic it. That equal gravity value surface IS “sea level“. If, according to gravity, any part of the sea was really 120m below another, water would flow there until equilibrium was reached.

Earth’s inertia (compared to everyday experience) is very large, and its rotation is very stable, because of its gigantic mass. Theories of significant rotation changes like the one you mention can never seem to come up with an energy source great enough to perpetrate this act.

“Alternative History” need only pass muster with the Scientific Method, and it won’t be “alternative” any longer.

Geophysics is far from my areas of expertise (Pleistocene Geology and Paleoclimatology), but I could probably offer some input to help your novel be more technically correct regarding matters of Earth Science.

You can contact me through my IAFI e-mail address on my previous reply.

Mark

(Mike, Thanks for the thanks.)
• Global Mapper User Trusted User
edited May 2011
Thanks Mark.
I appreciate you're willingness to help.
My interest -- as a lay person -- is not restricted to the novel: I'm also interested, because I think there is a lot to learn about pre-history.

I understand that the water in the oceans and the ice sheets are one -- just in different forms; hence sea level changes over time.
The Geopulsation Theory put forward refers to the apparent fact that the Earth's core rotates at a slightly different rate to the Earth's crust. Also the axis' are not quite the same. I've checked with other academic sources to see if the science community has anything to say on this matter -- it does. I think the energy source must be the effect resulting from the differences in core and crust rotation / axis. I'm not a scientist, so thats as far as I can go with that.

As part of my research / interest, I'm interested in what the environment in that region would have been like 12k calyr ago. I have some articles and papers, but the maps showing the paleo-vegetation are small and limited. Has anyone tried more regionalised and detailed maps. Also, I'm not entirely sure what the temperatures would be like compared to today.

Since we're heading out of GM territory here I'll e-mail you seperately after this, with various maps produced in GM if it helps.

One thing I have been trying to do with GM is use watershed generation to try and identify paleo-rivers on the Sunda Shelf. They are taking a lot of computer time and with mixed results. A test I tried on my home city was extremely disappointing, because it did not generate the rivers; so I wonder how accurate it is as a feature.

Be in contact via your e-mail.

Many thanks
Chris
edited May 2011
Even though you are headed out of GM territory, I really don't mind you continuing the conversation here. As a lay person myself (I'm a professional pilot by trade) I'm learning a lot and wouldn't mind seeing those screenshots of Global Mapper about the discussion. Feel free to continue...
• Global Mapper User Trusted User
edited May 2011
Paul, thanks for your interest. I wouldn‘t mind continuing on some about Geoids, Ellipsoids, elevation, sea level, etc. Though, as I said, Geodesy and Geophysics are not my forte. Chris and I can discuss pre-history privately.)

Chris,

I hope these paragraphs will help with some of the confusion you expressed, and at the same time be interesting to the forum. Please don’t be insulted if you already understand these things. The same goes for anyone else reading. I’ll respond to your e-mail asap.

Ellipsoids are (I think) theoretical perfect spheres with their shape adjusted for rotation (polar flattening) etc., and their origin coordinates adjusted for a “best fit” of the Earths surface in some particular region (mostly in the past), or for the whole Earth (now). [Mike?] The “Geoid” you are referring to is a map of the departures of an equal gravity value surface from some Ellipsoid; not a coordinate system. Although the Earth radius used for a specific Ellipsoid would seem to be like an “elevation” above it’s center, Ellipsoids are only horizontal coordinate systems.

[I’m guessing you may get different contours if you use different Ellipsoids, but it will only be because you have shifted your horizontal coordinate system on an irregular 3 dimensional surface.]

Sea level and therefore “elevation” (on the surface of a rotating sphere, no less) is not so absolute or as simple as one might first imagine. Just as the elevation of the ocean surface varies constantly over time, we now know that the whole solid surface of the Earth is continually undulating vertically. So, in nature, there is actually no unchanging vertical reference point (a “benchmark”). [Satellite based geodesy and altimetry may solve this, but gravity is a variable here, and it varies according to the uneven distribution of mass inside the Earth, which is not fully known yet.] Even the GPS CORS project will eventually succumb to tectonic vertical change. To this day, surveyors establish the vertical coordinate of their control point(s) by “ASSUMING a benchmark”. This works, has many advantages, and some disadvantages. As Paul knows, you always want to be on the vertical control system for where you actually are, not one from somewhere else.

“Level” and “flat” are not the same. Flatness refers to how closely a surface resembles a plane. Level-ness refers to how identical the elevation of the points in that plane are. A flat surface need not be level, but a level surface must be flat.

Paleo-rivers may well have existed in Sundaland, and this would be interesting to see. I have trouble with the watershed function also. At any particular scale, I can’t seem to figure out the right parameters without guessing at them and then “iterating” many times. I can’t seem to find an empirical relationship between the size of area I want to analyze, and the number of cells to enter, or the “drainage area” size.

Geodetic “Food for thought“:

As 6’ humans on the surface of an 8000 mile diameter planet, we live a very “horizontally oriented” existence. If I were to drive straight upwards the same distance I drive to take a commercial plane flight (75 miles); I would already be in space.

I once read that if you could shrink the Earth down to the size of an orange, keeping it’s surface relief to scale; that it would be smoother than an orange.

Mark
• Global Mapper User Trusted User
edited May 2011
Thanks Global_Flyer. I'll try and oblige. I'm about to attempt several screenshots...
1st = 3D from north(ish) looking down Java Sea -- Java is the island in front
2nd = watershed attempt to create paleorivers -- actually it is 5 attempts at watershedding superimposed. Despite the roughness, they provide a suprisingly better result than any of the individual attempts. Don't know how they measure up to more serious attempts at identifying paleo rivers on the now submerged Sunda Shelf. Comments welcome.3D Sunda Plain Java Sea from North 1.jpgPaleorivers 5watersheds combined 1.jpg
See if that worked....
They did -- anyway of making them larger?

Chris
• Global Mapper User Trusted User
edited May 2011
The 3D does not show generated contours. It is SRTM with custom shader.
• Global Mapper User Trusted User
edited May 2011
No legends because I'm not getting on well with GM legend. There is far too little control. I would essentially like to create my own legends from the components (I don't mean using script -- can't do that).
the next 2 are attempts at cross sections along the Java Sea using 3D profile in GM. The results of the profiles and mapping were captured and put into anther package (forget which one I used now -- probably PaintNet) to create image files.
Sunda Java Sea 02a.jpgSunda Java Sea 01a.jpg

Chris
• Global Mapper User Trusted User
edited May 2011
Mark,
Ellipsoids: they're not spheres -- I think those would be round. Someone correct me if I'm wrong. Ellipses are ellipses, but they do represent a perfect curved surface and were originally developed for global map making -- as a reference surface for horizontal and vertical dimensions. As I understand it, GPS also refers to WGS84 ellipsoid and then calculations are done to adjust the elevation data to the geoid. Again, I believe finding this discrepacy when GPS first came out eventually sparked off the development of satellites to measure gravity to prove / disprove the discrepacy. We know the results now and you can download the geoid and treat like other elevational data. Maybe I should create a screen shot of that too....
For some while now, I have been accumulating about 3 thick ring binders on cartography, geodesy, mapmaking, volcanism, earthquakes, sea level and more. Much I don't understand, but I get general 'pictures'...
What I really get confused about is all the different 'surfaces' and their relationships; hence my confusion about what height is actually measured against. I guess (?) it's what you need it for. Ships and aircraft certainly need the most accurate depth / height information and presumably that's against the geoid ????? ...perhaps thats not accurate enough either -- open to simple clarifications here.
My particular problem is relating the elevation results of SRTM, ASTER... to the better topographical maps (eg former Soviet topos -- which were generally of high standard -- otherwise mobile phone companies wouldn't be using ex-Soviet DEM for planning masts, etc.
I am aware that there is an error with the topos (it's published, and larger than you would expect), but I cannot find any consistent difference between the topo and the SRTM / ASTER results. ASTER (is it only topo and no bathymetric data?) is much better than SRTM (you're lucky in the States, because you get 30m resolution SRTM; whereas the rest of the World gets 90m!!!!). ASTER is 45m resolution. Contours generated from that follow the topo contours much better but still height differences -- form is quite good, height sucks.
I would whether I am causing myself unnecessary problems by thinking of the geoid -- any comments?

Level & Flat:
Good distinction -- hadn't thought of that one.
I'm sure you're absolutely correct from a technical point of view, but what happens when a none technical person is referring to something being level? It happens in Plato's text. I'm sure they are really thinking it's flat. -- comments?--
If so, that brings extra meaning to my cross-sections through the Java Sea.
Assuming we're talking of 'flatness'...
How flat (level?) has got to be before people start thinking it's not? This is not crazy.... When railway builders constructed them, at some point the gradient becomes irrelevant, because it is of no consequence to the train. When Brunel built the Great Western Railway (that's the UK) he used 1 in 1000 -- a very shallow gradient even for those times (1840's).
In short, when Plato referred to a large level plain, I'm sure it had a gradient and was not truly level (or flat). From memory I think the calculation I made for the Java Sea was in the order 1 in 10000 (yes ten thousand). The profile certainly doesn't look flat, but it is about 1100km long. Of more concern are the depressions in the 'plain' -- within red on the 3D image -- which would allow inundation into the plain area.
Since writers tend to use -120m contour they see no problem. At 11600calyr the sea level was anything from -43m to -67/68m, depending which academic paper you read. This is a huge difference to the area of land inundated.

Watershed: I'm glad I'm not alone with problems.
I tested a series of different resolutions (thanks Mike for that advice) and got varying results. The results of superimposing 5 of these sets can be seen in the screen shot. I actually think they produced something useful, despite their ragged appearance. The individual ones certainly did not. What you see in the image would really need trimming (ie many of the smaller water courses removing) to produce a better looking product.
How accurate they are is another question.
I tried doing something on my own city and they didn't even reproduce the main river -- full stop. A lesser river had some semblance to reality.
I've never let the computer run long enough at the higher resolutions to find out the better results.
I have i5-760, 8Gb RAM and 64bit W7P. I've just read an article about the benefits of SSD drives. The more going on the better the speed. Perhaps replacing C-drive with an SSD might speed up calculations. Anyone tried this?

Enough for now...
Chris
• Global Mapper User Trusted User
edited May 2011
Sundaland Paleorivers: different thicknesses to different watershed layers to better show differences.
Voris paleorivers underneath for reference.
Natural Earth rivers on current land.
2 major river sections not created by watershed generation.Sunda paleorivers 2labelled.pdf
edited May 2011
I think the missing river sections are due to depressions in the source DEM blocking the flow. Using a larger maximum depression fill depth should fix this, although it will make the watershed generation take potentially much longer. Hopefully soon I will be able to find a much faster depression filling algorithm to make watershed generation much faster.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com
• Global Mapper User Trusted User
edited May 2011
Mike,
Thanks -- appreciate you working on watershed issues...

Mark,
Before you go looking, see Sunda paleoveg -- the only info I can find in GIS format.
Is this as good as it gets?
Is it possible to add temperature info over this for different times periods (eg 11.6k BP)?

ChrisSunda paleoveg 1.jpg
edited May 2011
Chris,

I believe it is at the request of non-US governments that the full 30m resolution SRTM not be released for those parts of the world, at least not to everyone. It's really up to those other governments to allow the release so who knows.

The vertical datum for the SRTM data is EGM96 (horizontal is WGS84). I doubt they would update the vertical datum of the raw SRTM data since it's not being recollected, it would be up to software to do the conversion. See http://cddis.nasa.gov/926/egm96/egm96.html for more information on EGM96.

It would be great for the TanDEM-X data to be supported when acquired. I'm guessing it will be made available in some format that Global Mapper can use.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com
• Global Mapper User Trusted User
edited May 2011
Thanks Mike.
Interesting that SRTM is stuck at 90m when ASTER is available at 45m (?) resolution and not apparently stopped by governments.
I wonder what will happen with TanDEM-X.
It would be excellent news if you were able to get TanDEM-X added to online sources.

Also mention here, Mike -- that transparency slider you provided is excellent -- a major improvement -- thanks.
Chris
• Global Mapper User Trusted User
edited May 2011
I'll try posting this again....

ELEVATIONS, ELLIPSOIDS, GEOIDS, etc.

SRTM30 Plus
Ref.: http://eros.usgs.gov/#/Find_Data/Products_and_Data_Available/SRTM

Specification:
Projection: Geographic
Horizontal Datum: WGS84
Vertical Datum: EGM96 (Earth Gravitational Model 1996)
Vertical Units: Meters
Spatial Resolution: 1 arc-second for the United States
3 arc-seconds for global coverage
Raster Size: 1 degree tiles
C-band Wavelength: 5.8 cm

Questions:
Why is Level 2 (1 arc-second) not available outside USA?

What are the chances of it becoming available?

The specification (above) says the horizontal datum is EGM96. Does this mean that elevations from this grid are all from the geoid and not an ellipsoid?
I would very appreciate a clear clarification of this.

Is SRTM ever likely to be up-graded with EGM08?

Use:
Much quicker to use than ASTER, but poor resolution for contour generation.

Background:
Anyone interested…go to:
http://srtm.csi.cgiar.org/PDF/Jarvis4.pdf
“Practical use of SRTM data in the tropics – Comparisons with digital elevation models generated from cartographic data”, Andy Jarvis, Jorge Rubiano, Andy Nelson, Andrew Farrow and Mark Mulligan

This paper compares SRTM and GTOPO at 2 different resolutions..

ASTER
This provides better resolution (1.5 arc-seconds), but not below sea level -- at least in the Caspian Sea area.

Quality:
There are questions over the quality and consistency of data (re. http://www.ersdac.or.jp/GDEM/E/image/ASTERGDEM_ValidationSummaryReport_Ver1.pdf).

Image 1:
Shows anomalies in northern part of Caspian Sea -- satellite imaging rectangles clearly visible.
Image 1_ASTER anomalies screenshot 1_75%.jpg
Image 2:
Generating contours results in the anomalies being followed.
Image 2_ASTER anomalies screenshot 2_75%.jpg

Image 3:
Dagadzhik -- peninsular in Turkmenistan -- showing 26.1m contour using ASTER. Dagadzhik is on the southeast coast of the Caspian Sea. It follows satellite (Landsat 7) features closely. Similar exercises show closer consistency with topo map contours.
Image 3_Dagadzhik penin SRTM contour test 2 100mres_75%.jpg

Image 4:
The same cannot be said for SRTM30Plus. 26.1m contour, at 100m resolution. The only advantage, is a better shoreline creation.
Image 4_Dagadzhik penin SRTM contour test 2 100mres_75%.jpg

Note the considerable difference in accuracy.
Image 5:
South west part of the Volga Delta. Landsat 7 over ASTER. The ‘spotted dick’ (that’s a pudding with currants in for those who are thinking anything else!!) appearance of the contour -- 26.1m -- shows clearly, compared with a notional coastline of 1920. The results with SRTM were much further inland!
Image 5_VD SW ASTER contour sample 1.jpg

Issues:
No consistent coastline in very flat areas. Consists of a lot of small areas.

Satellite elevational data is not good at dealing with coastal areas.
When will it get better?

Trying to use ASTER at anything less than 100m resolution is painfully slow. Maybe faster broadband will help -- a little?????

TanDEM-X:
In an ideal World it would be great if GM could provide TanDEM-X data for download when it becomes available. 2m (relative) vertical accuracy.

http://www.infoterra.de/tandem-x_dem

Chris (2nd attempt)
• Global Mapper User Trusted User
edited May 2011
CORRECTION:
Images 3 and 4 are transposed. The first is SRTM. The second is ASTER.
• Global Mapper User
edited May 2015
Chris_ wrote: »
(p.s. - Why 11.6 ka ?)
Thanks Mark for replying; its much appreciated -- especially as my previous posts got no response what so ever.
And yes, you may well be able to help further.
-- Mike -- thanks for adding that --
I'm a retired (due to ill health) Local Government Officer (in the UK). I'm trying to write an alterntive history novel with many maps; also have started an historical atlas of the Russian revolutions and civil war (1917-1923). I have interests in history and pre-history. I don't believe we know that there has never been previous civilisation(s) that current knowledge has not indentified (in terms of main stream knowledge).
The AH novel has it's roots in Plato's Atlantis (don't stop reading now...I'm NOT about to go on about aliens and spacecraft!).
Of all the material I have read over the years, a large inundated area (post LGM) seems the most likely place (even if it is not Plato's Atlantis). So, I am trying to see how the physical description of the city and plain fit somewhere in Sundaland.

The late Professor Arysio Santos (Brazilian) wrote a book: 'Atlantis: The Lost Continent Finally Found'. It is certainly interesting -- so long as you set aside his style and definitive claim -- that sets out some interesting arguements.

Like a lot of people sharing the same locational view, they see -120m and the rectangular Java Sea and , vola it fits. They forget about sea level rise and one look at the Voris maps shows that 11.6k is touch and go for the plain to fit.
I'm just an amateur at this, so I might ask some 'obvious' questions.
I've created contour maps by shading SRTM30, by generating contours (1m interval), made cross sections using the 3D path tool, etc.
Area: to see if it fits the description
Cross-sections: to see how 'level' the bathymetry is -- when is level considered to be level? (in general terms).
The late Prof. also believes that Krakatua blew its top (VE7?) and initiated the destruction. I'm not convinced, because Plato does not mention volcanos directly, but does mention earthquakes. The is a description of what must be floating pumice.
The Prof. also refers to a modelling study by Keyes and someone else claiming that Krakatua exploded 535AD; but thinks they got the date wrong and says it was probably about the time I'm talking about.

I have just read another book by Richard W Welch, 'Roots of Cataclysm', that introduces a Geopulsation Theory. Its not entirely clear to me whether he's 'invented' this theory or not. Essentially, he claims that in addition to sea level changes relating to glaciation (eustacy?); that the planet also goes through cycles of rotational spin up and spin down (my term) which results in the ocean being pushed towards the Equator during spin-up -- centrifugal forces.
If there is any reality to this claim, then the sea level at Sunda could be a lot different. An example he uses refers to a -400m drop and the North Pole and a corresponding increase on +200m at the Equator! I think this is at the height of the LGM, so it would obviously be less as the ice melts.

I'm guessing that main stream geologists, geodesists, geographers and other scientists would find outlandish. How do you feel about these things?

I accept your point about ranges -- it makes sense -- even if I always want more.

I would still like to know the answer to the relationships of levels, because I am frustrated at not being able to find the answer.

The question is also relevant to the novel research in another way -- the Caspian Sea.

Another source was: Eden in the East, Stephen Oppenheimer.
Indeed my interest in sea level stuff is World wide, including the Persian Gulf and North Atlantic.
My overall time frame of interest goes back 130k+ cal yrs.

So, Mark -- if you're still with me -- perhaps this is a discussion you might want to continue via direct e-mail -- if that can be arranged.
Any help would be most appreciated.

Chris

Hi Chris,

I just found your post here. You have the same interest with me. Regarding the Atlantis, I made some maps with different sea water level. I have written a book titled "Atlantis: The lost city is in Java Sea", available in Amazon as a Kindle eBook.

Here are some references to my book:
Amazon.com: Atlantis: The lost city is in Java Sea eBook: Dhani Irwanto: Kindle Store
Book summary: Atlantis, the lost city is in Java Sea

Regards,
Dhani Irwanto
• Global Mapper User Trusted User
edited May 2015
Hi Chris,

I just found your post here. You have the same interest with me. Regarding the Atlantis, I made some maps with different sea water level. I have written a book titled "Atlantis: The lost city is in Java Sea", available in Amazon as a Kindle eBook.

Here are some references to my book:
Amazon.com: Atlantis: The lost city is in Java Sea eBook: Dhani Irwanto: Kindle Store
Book summary: Atlantis, the lost city is in Java Sea

Regards,
Dhani Irwanto

Hello,
That was a surprise post! Thank you for contacting me.
Sorry for the delay, but I have been in hospital and now recovering.
I have printed the summary to read. I can't promise a quick reply due to health issues.
As it happens I am currently reading 'Earth Underfire: Humanity's Survival of the Ice Age', by Paul A. LaViollette, PhD

I'm afraid I have done no more since the time of the above posts.

Perhaps we can stay in cantact via e-mail?

Best wishes

Chris
• Global Mapper User
edited May 2015
Hi Chris,

Thanks for the reply and for reading my article. Hopefully you can be totally recovered of the disease and become well.

Thanks to GlobalMapper, I can find a lot of things that a few can imagine, from the now mostly free digital elevation grids, not only above sea water but also below. The above work is what I found using GlobalMapper.

If you would like to contact me via e-mail, I will not mind. Here is my email: dhani_irwanto@indonesia-hydro.com.

Best wishes,
Dhani
• Global Mapper User Trusted User
Sorry for reviving an old post...

Now that the TANDEM-X DEM is freely available (referenced to an ellipsoid; https://geoservice.dlr.de/web/dataguide/tdm90/), how can I use Global Mapper to transform TANDEM-X elevation data to the Vertical Datum EGM96, i.e., to be able to use Tandem-X data and SRTM in parallel.

Thank you!

Sebastien
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Thank you for your answer but I don't see how to implement this in Global Mapper. I am currently trying to convert the DEM using VDATUM: https://vdatum.noaa.gov/welcome.html
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seb3343 said:
Thank you for your answer but I don't see how to implement this in Global Mapper. I am currently trying to convert the DEM using VDATUM: https://vdatum.noaa.gov/welcome.html
Update: VDATUM worked perfectly. And it's free. I am glad I didn't need to invest in Geographic Calculator just to process one DEM ;-)