# Xyz2grid latitude/longitude definitions & choosing the number of levels in grd2cpt **URL:** \ **Category:** GMT.jl Q&A\ **Created:** [May 29, 2024, 11:45am UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931 "2024-05-29T11:45:42Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![Cornec\_H](https://avatars.discourse-cdn.com/v4/letter/c/2bfe46/32.png) [@Cornec\_H](https://forum.generic-mapping-tools.org/u/Cornec_H)\ **Post date:** [May 31, 2024, 11:04am UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/21 "2024-05-31T11:04:38Z")
I also tried using `makecpt` to create a cpt with more bins. I am only plotting 25 bins because it seems that trying to do more currently terminates the Julia REPL. ```auto using StatsBase gdp_quantiles=[percentile(gdpneti, 4i) for i in 1:25] gdp_grd = xyz2grd(gdp_df, region=(-175.5, 175.5, -55.5, 83.5), spacing=(1.0, 1.0), reg=:g) C=makecpt(cmap=:polar, range=gdp_quantiles, no_bg=true) coast(water="lightblue", region=(-180, 180, -60, 75), proj=:equidistCylindrical) grdimage!(gdp_grd, title = "GDP Distribution", nan_alpha=true, colorbar = true, savefig ="hist_gdp.jpg") ``` The documentation provides the following example: ```auto imshow(makecpt(cmap=(:red,:green,:blue), range=[0,100,300,1000], no_bg=true), horizontal=true) ``` However, this does not in fact create the kind of cpt I want. The map now only has two colors and looks like this: ![hist_gdp](https://canada1.discourse-cdn.com/flex047/uploads/gmt/original/2X/4/43390155c7143793a158a588fa12605d42029838.jpeg) I feel like I am fundamentally missing something about how this works. Also any indication about how to tell the colorbar to plot the colorscale evenly instead of trying to force equidistant values would be super appreciated. ---
**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\ **Post date:** [May 31, 2024, 11:26am UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/22 "2024-05-31T11:26:40Z")
OK, from the beginning. (prefer GMT methods to load the data) ```auto Dgdp = gmtread("gdp_test_df.txt") BoundingBox: [-179.5, 179.5, -88.5, 83.5, 0.0001, 753.4839] 19235×3 GMTdataset{Float64, 2} Row ¦ col.1 col.2 col.3 -------+------------------------- 1 ¦ 139.5 35.5 753.484 2 ¦ 135.5 34.5 400.838 ``` From the `BoundingBox` (with a little experience) we can guess immediately that the data begs for a pixel registration. So: ```auto G = xyz2grd(Dgdp, region=(-180,180,-89,84), reg=:p, inc=1); ``` Now the difficulties arise from the data itself. Whoever made it decided many times to assign the GDP to countries capitals and _country fields_ became very poor. Furthermore, Tokyo is twice richer than the next contender (some capital in Europe). All this to say that the data spans 4 orders of magnitude so only a logarithmic color scale can save you. And here the `polar` CPT is no good (a likely GMT bug). ```auto C = makecpt(range=(0.0001,3), log=true, background=true); viz(G, C=C, colorbar=true) ``` ![gdp](https://canada1.discourse-cdn.com/flex047/uploads/gmt/original/2X/7/7bbe2f69e49ffc969a5380f63c3a750cc42501f2.jpeg) It’s very pixelated, right. You can resample it (now I see why you tried grdsample before) but it only get a little better. ```auto Gsamp = grdsample(G, inc=0.25); viz(Gsamp, ...) ``` … add `coast` at your will Note, a xyz2grd GMT feature sets a bad projection info in the grid. If you want to make maps with automatic projections or else, better do this manual fix. ```auto G.proj4 = "+proj=longlat"; ``` ---
**Author:** ![PlanetGus](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/planetgus/32/303_2.png) [@PlanetGus](https://forum.generic-mapping-tools.org/u/PlanetGus)\ **Post date:** [May 31, 2024, 11:32am UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/23 "2024-05-31T11:32:50Z")
> [@Cornec\_H](#): > > I want to be able to customize the amount of histogram bins that `grd2cpt` uses to make a cpt, so that I can show more variation in my data Well. I can’t tell you much more as I am not trying it myself, but as I understand your problem, the “number of bins” is `-I` with `-Cn` to just have the number of values per bin. Eventually `-Cz` for the sum of the values ? ---
**Author:** ![Cornec\_H](https://avatars.discourse-cdn.com/v4/letter/c/2bfe46/32.png) [@Cornec\_H](https://forum.generic-mapping-tools.org/u/Cornec_H)\ **Post date:** [May 31, 2024, 1:37pm UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/24 "2024-05-31T13:37:02Z")
Thank you @Joaquim, that actually makes a lot of sense! I think I have got it 99% working now (only the colorbar left to fix) I only now understood how to use the `nlevels` option in `grd2cpt`. To prevent `nlevels` from defaulting to equal-distance slices, I need to write `nlevels="nlevels+c", I had tried `nlevels=nlevels+“c” but this doesn’t work. Sorry if I have been asking a lot of questions, things are slowly falling into place. This piece of code colors the map in the way I want: ```auto gdp_grd = xyz2grd(gdp_df, region=(-175.5, 175.5, -55.5, 83.5), spacing=(1.0, 1.0), reg=:g) C = grd2cpt(gdp_grd,cmap=:polar,nlevels="128+c") coast(water="lightblue", region=(-180, 180, -60, 75), proj=:equidistCylindrical) colorbar!(region=(-180, 180, -60, 75), equal_size=true, pos=(justify=:CT, size=(5,0.2), offset=(1,0.25)), cmap=C) grdimage!(gdp_grd, title = "GDP Distribution", nan_alpha=true,savefig ="hist_gdp.jpg") ``` ![hist_gdp](https://canada1.discourse-cdn.com/flex047/uploads/gmt/original/2X/7/7f1f9b0f60566f1516ff65734f4bb8fced01e904.jpeg) However, now I am trying to not have annotations for every color slice. I have to use `-L` to make it so that the colorbar is readable (given the spread of the data), but then I cannot edit ticks and annotations using `B` without getting the following error message: ```auto psscale [ERROR]: Option -L: Cannot be used if -B option sets increments. ``` How would I go about modifying the colorbar so that it only has ticks and annotations at certain z-values? I have the numeric values of the deciles: ```auto 0.0016 0.0063 0.015419999999999982 0.0327 0.0687 0.14753999999999998 0.3457 0.8781000000000003 2.5724199999999984 753.4839 ``` Or is this impossible when I set `-L=true`? Again, big thank you to everyone who has been helping out with answers! And let me know if I should just open up a new topic about the annotation issue since my original questions for this thread are now solved (not sure what the best practice is for that). ---
**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\ **Post date:** [May 31, 2024, 3:06pm UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/25 "2024-05-31T15:06:40Z")
You are trying to escape to the 4 orders of magnitude issue … but the only escape is an eventually good color map created manually. This my look better (the figure, not the colorbar) but IMY or much more misleading than the one with logarithmic color scale. (and yes, I forgot (didn’t reread the docs) that nlevels alone makes grd2cpt behave like makecpt) ```auto C = grd2cpt(G,cmap=:polar,nlevels="64+c"); viz(gdp_grd, C=C, coast=(water="lightblue",), colorbar=true) ``` ![gdp](https://canada1.discourse-cdn.com/flex047/uploads/gmt/original/2X/e/ea0414799fa8bc767634091257474f98ccf7f1ba.jpeg) But now the colorbar seams to have a single color, but it doesn’t ```auto C Extract of a GMTcpt exposed as a GMTdataset for display. Model: rgb Color depth: 24 11×9 GMTdataset{Float64, 2} Row │ r1 g1 b1 r2 g2 b2 alpha z1 z2 ─────┼───────────────────────────────────────────────────────────────────────── 1 │ 23.0 23.0 255.0 23.0 23.0 255.0 0.0 0.0001 0.0017 2 │ 70.0 70.0 255.0 70.0 70.0 255.0 0.0 0.0017 0.0062 3 │ 116.0 116.0 255.0 116.0 116.0 255.0 0.0 0.0062 0.0143 4 │ 162.0 162.0 255.0 162.0 162.0 255.0 0.0 0.0143 0.0289546 5 │ 209.0 209.0 255.0 209.0 209.0 255.0 0.0 0.0289546 0.0557 6 │ 255.0 255.0 255.0 255.0 255.0 255.0 0.0 0.0557 0.110282 7 │ 255.0 209.0 209.0 255.0 209.0 209.0 0.0 0.110282 0.220736 8 │ 255.0 162.0 162.0 255.0 162.0 162.0 0.0 0.220736 0.492939 9 │ 255.0 116.0 116.0 255.0 116.0 116.0 0.0 0.492939 1.1358 10 │ 255.0 70.0 70.0 255.0 70.0 70.0 0.0 1.1358 3.10747 11 │ 255.0 23.0 23.0 255.0 23.0 23.0 0.0 3.10747 753.484 ``` There is a way to force the annotation to be on the color slices transitions but I think it implies saving the CPT on disk and edit. Don’t remember to have any GMT.jl helper syntax for that. ---
**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\ **Post date:** [May 31, 2024, 3:21pm UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/26 "2024-05-31T15:21:22Z")
I think this one shows better what is happening ```auto C = grd2cpt(G,cmap=:polar,nlevels="12+c"); viz(G, C=C, coast=(water="lightblue",), colorbar=(C=C, L=true)) ``` ![gdp](https://canada1.discourse-cdn.com/flex047/uploads/gmt/original/2X/3/31b1f64f2f809dc01cd9c8487dcbc04789d43810.jpeg) ---
**Author:** ![Cornec\_H](https://avatars.discourse-cdn.com/v4/letter/c/2bfe46/32.png) [@Cornec\_H](https://forum.generic-mapping-tools.org/u/Cornec_H)\ **Post date:** [May 31, 2024, 3:23pm UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/27 "2024-05-31T15:23:42Z")
Hmm, I see your point. Is there maybe a way to suppress all annotations and ticks on the colorbar in my colorbar function? I tried using `frame=(noframe=true)` but that does not seem to work as I thought it would. ```auto colorbar!(region=(-180, 180, -60, 75), frame=(noframe=true),equal_size=true,nolines=true,pos=(justify=:CT, size=(5,0.2), offset=(1,0.25)),cmap=C) ``` If there is nothing else written besides the colorbar, I could add the correct annotations separately in a different program. Not very clean, but better than either a monochrome colorbar or \>60 overlapping annotations ---
**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\ **Post date:** [May 31, 2024, 3:32pm UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/28 "2024-05-31T15:32:07Z")
> [@Cornec\_H](#): > > `frame=(noframe=true)` This is normally a Julia mistake. To make it a NamedTuple it has to have at least a ending colon `frame=(noframe=true,)` > [@Cornec\_H](#): > > I could add the correct annotations separately in a different program. This is not a software issue, it’s a data issue. To have histogram equalization and few data annotations you can not do much better (except having less decimals) than my last example. And you have to ask yoursel if you want to have almost the same color for GDPs in [1.1 3.1] as in [3.1 753.5]. ---
**Author:** ![Cornec\_H](https://avatars.discourse-cdn.com/v4/letter/c/2bfe46/32.png) [@Cornec\_H](https://forum.generic-mapping-tools.org/u/Cornec_H)\ **Post date:** [May 31, 2024, 4:02pm UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/29 "2024-05-31T16:02:23Z")
> [@Joaquim](#): > > This is normally a Julia mistake. To make it a NamedTuple it has to have at least a ending colon `frame=(noframe=true,)` Oh, I see. But this does not prevent the annotations and ticks from showing up either. The colorbar annotations are left unchanged. ---
**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\ **Post date:** [May 31, 2024, 4:27pm UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/30 "2024-05-31T16:27:35Z")
Will have to see. GMT itself does not allow not plotting the axes. That `noframe` is a trick made by plotting them fully transparent. Maybe it is skipped when plotting colorbars. ---
**Author:** ![Cornec\_H](https://avatars.discourse-cdn.com/v4/letter/c/2bfe46/32.png) [@Cornec\_H](https://forum.generic-mapping-tools.org/u/Cornec_H)\ **Post date:** [May 31, 2024, 4:58pm UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/31 "2024-05-31T16:58:00Z")
Thanks for checking it out! I really appreciate all the help I was also looking at `frame=:noannot` (if that is the correct syntax) but that option also doesn’t work when using `-L`. ---
**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\ **Post date:** [May 31, 2024, 5:32pm UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/32 "2024-05-31T17:32:56Z")
Right, GMT does not allow it, and rightful I would say. What is the meaning of a colorbar without annotations? EDIT: but if you insist you can make them invisibly smal. _e.g._ ```auto colorbar=(C=C, L=true, par=(FONT_ANNOT=0.1,)) ``` ---
**Author:** ![Cornec\_H](https://avatars.discourse-cdn.com/v4/letter/c/2bfe46/32.png) [@Cornec\_H](https://forum.generic-mapping-tools.org/u/Cornec_H)\ **Post date:** [May 31, 2024, 8:35pm UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/33 "2024-05-31T20:35:27Z")
That actually gets me most of the way there. I understand that this is not the cleanest way but: ```auto gdp_grd = xyz2grd(gdp_df, region=(-175.5, 175.5, -55.5, 83.5), spacing=(1.0, 1.0), reg=:g) C = grd2cpt(gdp_grd,cmap=:rainbow, nlevels="128+c") C2 = grd2cpt(gdp_grd,cmap=:rainbow,nlevels="8+c") coast(water="lightblue", region=(-180, 180, -60, 75), proj=:equidistCylindrical) colorbar!(pos=(justify=:CT, size=(5,0.2), offset=(1,0.25)),region=(-180, 180, -60, 75), equal_size=true,nolines=true,cmap=C2) colorbar!(par=(FONT_ANNOT=0.1,),region=(-180, 180, -60, 75), equal_size=true, nolines=true,pos=(justify=:CT, size=(5,0.2), offset=(1,0.25)),cmap=C) grdimage!(gdp_grd, title = "GDP Distribution", nan_alpha=true,savefig ="hist_gdp.jpg", cmap=C) ``` I basically have a colorbar with fewer annotations (or `nlevels`) in the layer below and then overlaying it with the colormap used in grdimage but hiding the 128 annotations to get something that is closer to the map I was trying to emulate that I shared in the beginning. Not elegant, but it gets the job done. This creates the following map: ![hist_gdp](https://canada1.discourse-cdn.com/flex047/uploads/gmt/original/2X/8/8ea2b7594ec9325767af205aeb22b46d4285ef3a.jpeg) If I can now make the ticks disappear and maybe reduce the number of decimal places that would be ideal, but this is already pretty good. Still surprised that I cannot simply define custom annotations at specific values to override the automatic annotation at each color slice (something like `frame=(zticks=([0.0, 0.0101, 0.0687, 0.5323, 753.4839], ["0.0", "0.0101", "0.0687", "0.5323", "753.4839"]`), but this is an okay workaround. Thanks a lot for all the help. ---
**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\ **Post date:** [May 31, 2024, 9:55pm UTC](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931/34 "2024-05-31T21:55:36Z")
You can configure mostly everything in GMT, but deeper and less used things require diving into the GMT documentation. All parameters are set via gmt.conf (the par=(…) in GMT.jl). See this for tick lengths [https://docs.generic-mapping-tools.org/dev/gmt.conf.html#term-MAP\_TICK\_LENGTH](https://docs.generic-mapping-tools.org/dev/gmt.conf.html#term-MAP_TICK_LENGTH) GMT.jl has this examples for more elaborated colormaps [https://www.generic-mapping-tools.org/GMTjl\_doc/examples/CPTs/01\_cpt\_hinge/#example\_1336622456775633275](https://www.generic-mapping-tools.org/GMTjl_doc/examples/CPTs/01_cpt_hinge/#example_1336622456775633275) And finally, this describes the CPT structures in depth and how one may decide what to annotate [https://docs.generic-mapping-tools.org/dev/reference/features.html#color-palette-tables](https://docs.generic-mapping-tools.org/dev/reference/features.html#color-palette-tables) And, if I may say so, my opinion is that you are weighting to much the aesthetic side of the figure in detriment of its information content. [Previous page](https://forum.generic-mapping-tools.org/t/xyz2grid-latitude-longitude-definitions-choosing-the-number-of-levels-in-grd2cpt/4931.md?page=1)