Skip to content

Commit 5f82203

Browse files
andypbarrettandypbarrett
authored
Merge pull request #7 from nsidc/geographic-to-image_coords-howto
Geographic to image coords howto
2 parents 8325cb1 + f933202 commit 5f82203

File tree

4 files changed

+181
-12
lines changed

4 files changed

+181
-12
lines changed

‎_quarto.yml‎

Lines changed: 19 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -24,7 +24,25 @@ website:
2424
collapse-level: 1
2525
contents:
2626
- text: "Welcome"
27-
href: nsidc-data-cookbook/introduction.qmd
27+
href: index.qmd
28+
- text: "Our Cookbook"
29+
href: nsidc-data-cookbook/our-cookbook.qmd
30+
- text: "Tools we use"
31+
href: tools-we-use.qmd
32+
- text: "Data Structures - types of data"
33+
href: reference-guides/data-structures.qmd
34+
- text: "Dataset search and discovery"
35+
href: reference-guides/dataset-search-and-discovery.qmd
36+
- text: "Common NSIDC file formats"
37+
href: reference-guides/nsidc-file-formats.qmd
38+
- text: "NSIDC map projections and grids"
39+
href: nsidc-data-cookbook/reference-guides/projections.qmd
40+
- text: "Plotting NSIDC Data"
41+
href: nsidc-data-cookbook/reference-guides/plotting-data.qmd
42+
- text: "Datasets"
43+
href: nsidc-data-cookbook/reference-guides/datasets.qmd
44+
- text: "What is an API?"
45+
- text: "NASA collections and granules"
2846
- section: "How do I..."
2947
href: nsidc-data-cookbook/how-to-guides/overview.qmd
3048
contents:
@@ -35,17 +53,6 @@ website:
3553
href: nsidc-data-cookbook/how-to-guides/netcdf_cf.qmd
3654
- text: "get the latitude and longitudes for grid cells"
3755
href: nsidc-data-cookbook/how-to-guides/get_latitude_and_longitude.qmd
38-
- section: "Tutorials"
39-
contents:
40-
- section: "Reference Guides"
41-
href: nsidc-data-cookbook/reference-guides/overview.qmd
42-
contents:
43-
- text: "What is an API?"
44-
- text: "NASA collections and granules"
45-
- text: "Map projections and grids"
46-
href: nsidc-data-cookbook/reference-guides/projections.qmd
47-
- section: "Examples"
48-
contents:
4956

5057
format:
5158
html:

‎nsidc-data-cookbook/introduction.qmd‎ renamed to ‎index.qmd‎

File renamed without changes.

‎nsidc-data-cookbook/how-to-guides/get_column_and_row_indices.ipynb‎

Lines changed: 151 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,151 @@
1+
{
2+
"cells": [
3+
{
4+
"cell_type": "raw",
5+
"id": "f783c0ec-fe8a-41c3-8997-9d4a70a9a373",
6+
"metadata": {},
7+
"source": [
8+
"---\n",
9+
"title: \"How to get grid column and row indices for latitude and longitude\"\n",
10+
"author: Andrew P. Barrett\n",
11+
"date: last-modified\n",
12+
"---"
13+
]
14+
},
15+
{
16+
"cell_type": "markdown",
17+
"id": "676acb3d-0bbe-456e-b9ee-c69a3c8ed5be",
18+
"metadata": {},
19+
"source": [
20+
"## Problem\n",
21+
"\n",
22+
"I want to get the column and row indices of a grid that corresponds to latitude and longitude. "
23+
]
24+
},
25+
{
26+
"cell_type": "markdown",
27+
"id": "12a5c03b-05ea-4f1d-935a-4c302384fce5",
28+
"metadata": {},
29+
"source": [
30+
"## Solution\n",
31+
"\n",
32+
"In Python you can use the `pyproj` package to transform latitude and longitude (geographic coordinates) to x and y (projected coordinates) in the EASE Grid CRS, and then use the `affine` package to get the column and row indices (`(i,j)` image coordinates)."
33+
]
34+
},
35+
{
36+
"cell_type": "code",
37+
"execution_count": 5,
38+
"id": "99e45ce1-3e02-4525-8722-7056888d4839",
39+
"metadata": {},
40+
"outputs": [
41+
{
42+
"name": "stdout",
43+
"output_type": "stream",
44+
"text": [
45+
"414.4236938226952 205.11385157648664\n"
46+
]
47+
}
48+
],
49+
"source": [
50+
"from pyproj import Transformer\n",
51+
"from affine import Affine\n",
52+
"import numpy as np\n",
53+
"\n",
54+
"# Define the coordinate transform from WGS84 (lat, lon) \n",
55+
"# to CRS of your grid using EPSG codes.\n",
56+
"# WGS84 is EPSG:4326\n",
57+
"# NSIDC Northern Hemisphere EASE Grid is EPSG:3408\n",
58+
"transformer = Transformer.from_crs(4326, 3408)\n",
59+
"\n",
60+
"# Define the affine transformation matrix\n",
61+
"grid_cell_width = 50057.094722222224\n",
62+
"grid_cell_height = -50057.094722222224\n",
63+
"upper_left_x = -9036842.76\n",
64+
"upper_left_y = 9036842.76\n",
65+
"geotransform = Affine(grid_cell_width, 0.0, upper_left_x,\n",
66+
" 0.0, grid_cell_height, upper_left_y)\n",
67+
"\n",
68+
"x, y = transformer.transform(-45., 84.) # latitude and longitude\n",
69+
"i, j = ~geotransform * (x, y) # ~geotransform is the inverse operation\n",
70+
"\n",
71+
"print(i,j)"
72+
]
73+
},
74+
{
75+
"cell_type": "markdown",
76+
"id": "ca295ab6-44e3-48de-a4fa-62accf6694dd",
77+
"metadata": {},
78+
"source": [
79+
"`i` and `j` are in image coordinates with the origin as the upper-left corner of the upper-left grid-cell. To get the column and row indices of the cell you can use `floor`."
80+
]
81+
},
82+
{
83+
"cell_type": "code",
84+
"execution_count": 9,
85+
"id": "b3fa03fc-c38e-48e5-bff3-2d30b9f8e5d2",
86+
"metadata": {},
87+
"outputs": [
88+
{
89+
"name": "stdout",
90+
"output_type": "stream",
91+
"text": [
92+
"414 205\n"
93+
]
94+
}
95+
],
96+
"source": [
97+
"col, row = np.floor(i).astype(int), np.floor(j).astype(int)\n",
98+
"print(col, row)"
99+
]
100+
},
101+
{
102+
"cell_type": "markdown",
103+
"id": "98617747-c928-4a1f-8b63-1635d41546da",
104+
"metadata": {},
105+
"source": [
106+
"## Discussion\n",
107+
"\n",
108+
"Many grids, whether they are images or gridded data are in _projected coordinate systems_ so that they can be displayed on a 2D screen. Converting from geographic coordinates (_latitude_, _longitude_) to image coordinates (_column_, _row_) is usually a two step process. In the first step, the geographic coordinates are transformed from a Geographic Coordinate System with coordinates _latitude_ and _longitude_ to the _projected coordinate system_ of the image or grid. This is a _Cartesian_ coordinate system with coordinates _x_ and _y_, usually in meters. In the second step, the _projected coordinates_ are converted into _image coordinates_. This is another _Cartesian_ coordinate system with coordinates _column_ and _row_ or _i_ and _j_ with units grid-cells. Image coordinate systems often have the origin in the upper-left corner of the upper-left grid cell.\n",
109+
"\n",
110+
"In Python the first step is easily acheived using the `pyproj` package. If the Coordinate Reference System (CRS) of the grid is registered in the EPSG database, defining the transformation is easily done with the EPSG codes for the CRS for the _latitude_ and _longitude_ coordinates and the CRS of the destination grid. Usually, _latitude_ and _longitude_ are in the Ensemble WGS84 CRS, which is EPSG:4326. For NSIDC gridded data, the CRS is some flavour of EASE Grid or Polar Stereographic. See the help pages for [EASE Grid](https://nsidc.org/data/user-resources/help-center/guide-ease-grids) and [Polar Stereographic](https://nsidc.org/data/user-resources/help-center/guide-nsidcs-polar-stereographic-projection) for more information. \n",
111+
"\n",
112+
":::{note}\n",
113+
"Both EASE Grid and Polar Stereographic CRS come in two main flavours _Original_ and WGS84. For EASE Grids, these two flavours are EASE-Grid and EASE-Grid v2.0.\n",
114+
":::\n",
115+
"\n",
116+
"TODO: Describe projected to image coordinate conversion\n",
117+
"\n",
118+
"Add image showing the relationship between the three coordinate systems"
119+
]
120+
},
121+
{
122+
"cell_type": "code",
123+
"execution_count": null,
124+
"id": "9d688cd2-1b28-4dba-851c-fae51a476c46",
125+
"metadata": {},
126+
"outputs": [],
127+
"source": []
128+
}
129+
],
130+
"metadata": {
131+
"kernelspec": {
132+
"display_name": "Python 3 (ipykernel)",
133+
"language": "python",
134+
"name": "python3"
135+
},
136+
"language_info": {
137+
"codemirror_mode": {
138+
"name": "ipython",
139+
"version": 3
140+
},
141+
"file_extension": ".py",
142+
"mimetype": "text/x-python",
143+
"name": "python",
144+
"nbconvert_exporter": "python",
145+
"pygments_lexer": "ipython3",
146+
"version": "3.12.3"
147+
}
148+
},
149+
"nbformat": 4,
150+
"nbformat_minor": 5
151+
}

‎our-cookbook.qmd‎

Lines changed: 11 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,11 @@
1+
---
2+
title: "Our Cookbook"
3+
---
4+
5+
## How to use this cookbook
6+
7+
8+
## Get Started
9+
10+
11+
## Tools we use

0 commit comments

Comments
 (0)