1. INTRODUCTION
The Department of Posts has undertaken an initiative to establish a Digital Public Infrastructure (DPI) for a standardized, geo-coded addressing system in India. As a part of this initiative, the Department is releasing final version of DIGIPIN – the foundation layer of the DPI. This initiative seeks to provide simplified addressing solutions for seamless delivery of public and private services and to enable "Address as a Service" (AaaS) across the country.
DIGIPIN is an open-source national level addressing grid developed by Department of Posts in collaboration with IIT Hyderabad and NRSC, ISRO and is a key component of the digital address ecosystem. The beta version of the technical document on DIGIPIN was placed for public comments and expert opinion.
After thorough analysis of the comments received through various stakeholder consultations, the Department has now finalized the DIGIPIN Grid that incorporates the relevant inputs. The DIGIPIN layer will serve as a uniform address referencing framework available both offline and online, enabling logical and precise location identification with directional attributes across India, offering significant advantages of ensuring precise geographic identification and thus complementing the conventional addressing system by providing an additional attribute. This would bridge the gap between physical locations and their digital representation.
2. DESIGN APPROACH
2.1 Core Concept:
The DIGIPIN layer is the cornerstone of the entire digital address ecosystem.
DIGIPIN is visualised as an alpha numeric offline grid system that divides the geographical territory of India into uniform 4-meter by 4-meter(approx.) units. Each of these 4m X 4m units (approx.) is assigned a unique 10-digit alphanumeric code, derived from the latitude and longitude coordinates of the unit. This alphanumeric code serves as the offline addressing reference for any specific location within the DIGIPIN system. DIGIPIN is thus strictly a function of the latitude and longitude of the location represented as a grid value. The system is designed to be scalable, adaptable, and integrated with existing GIS applications.
2.2 DIGIPIN layer:
DIGIPIN layer will act as the addressing reference system which will be available offline and can be used for locating addresses in a logical manner with directional properties built into it due to the logical naming pattern followed in its construction. DIGIPIN Grid system being an addressing referencing system, can be used as the base stack for development of other ecosystems where addressing is one of the processes in the workflow. Since DIGIPIN solely represents a location and does not store any personal information, it respects privacy.
3. DIGIPIN: Code Architecture
The detailed structure is such that the DIGIPIN is essentially an encoding of the latitude and longitude of the address into a sequence of alphanumeric symbols using the following 16 symbols: 2, 3, 4, 5, 6, 7, 8, 9, C, J, K, L, M, P, F, T.
The process of identifying the cells is done in a hierarchical fashion. The encoding is performed at various levels, and the basic idea is the following:
- A bounding box is used that covers the entire country.
- The bounding box is split into 16 (i.e., 4x4) regions. Each region is labeled by one of symbols 2, 3, 4, 5, 6, 7, 8, 9, C, F, J, K, L, M, P, T. The first character in the code would identify one of these regions. This is called the level-1 partition.
- Each region is then subdivided into 16 subregions in a similar fashion. Each of the 16 subregions are labeled by the 16 characters. For a given region, the subregion is identified by the second character of the code. Therefore, the first two characters of the code uniquely identify one of the 16^2=256 subregions. This is called the level-2 partition.
- The encoding of successive characters, and therefore the next 8 levels is done in an identical fashion. The 10-symbol code therefore uniquely identifies one of the 16^10 cells within the bounding box.
3.1 Bounding box:
- Longitude 63.5 – 99.5 degrees east
- Latitude 2.5 – 38.5 degrees north
- The Coordinate Reference System (CRS) used in the proposed code design is EPSG:4326. Using EPSG:4326 (WGS84 datum at epoch 2005) has several advantages like wide recognition and adoption, simplicity and global coverage.
The choice of the corner points of the bounding box are based on the following considerations:
- This includes the entire territory of India.
- Since the proposed extent has latitudinal and longitudinal extent of 36°, the final grid size after 10th iteration is a perfect square and has smaller size of 3.8m x 3.8m.
- This bounding box ensures alignment of DIGIPIN level-1 grid lines with Survey of India's 0.25° x 0.25° toposheets (topographical sheet numbering system), which are used for mapping at 1:50K scale.
- Includes the maritime Exclusive Economic Zone (EEZ), and therefore DIGIPIN allows to provide addresses to Indian assets in the sea (oil rigs, future man-made islands, etc.), or even potentially be used to locate regions in the sea by the maritime sector. The maritime EEZ is computed assuming 200 nautical miles extent from the coastline.
- Level-1 grid lines do not cut through cities with very large population.
- The level-10 cells would be almost rectangular, but the dimensions would vary based on the latitude. This would translate to a cell of size smaller than 3.8-meter x 3.8-meter if measured at the equator, and this is reasonable given the accuracy of most current commercially available Global Navigation Satellite System (GNSS).
3.2 Properties of DIGIPIN
- DIGIPIN contains the geographic location of the area. It is possible to extract the latitude and longitude of the address from the DIGIPIN with low complexity.
- DIGIPIN has been designed for the Indian context. All points of interest to India (including maritime regions) can be assigned codes, and it is possible to assign code even in areas that are very densely populated.
- The format of the DIGIPIN is intuitive and human-readable. Effort was made to infuse a sense of directionality within the format of DIGIPIN.
- DIGIPIN is independent of the land use pattern and the structure built. Note that DIGIPIN is designed as a permanent digital infrastructure, that does not change with changes in the names of state, city or locality, or with changes in the road network in an area.
- The length of the DIGIPIN is designed to be as small as possible in order to provide an efficient digital representation of addresses.
3.3 Labelling of regions at various levels
- In first iteration (level-1), the bounding box is divided into 16 regions using a 4x4 grid and each region is labelled using 16 symbols, 2, 3, 4, 5, 6, 7, 8, 9, C, F, J, K, L, M, P, and T, as shown in Figure 2.
- A consistent gridding and labelling scheme is applied across all subsequent iterations (Levels 2 to 10). For these levels, the same labelled grid is utilized, with the labelling process carried out in a hierarchical manner.
- Each Level-1 region is further split into 16 sub-regions called Level-2 regions as illustrated in the Figure 3. The regions are hierarchically partitioned into sub-regions in an identical fashion.
- Symbols are assigned in anticlockwise fashion, spiraling outwards. The grid provides some sense of directionality and adjacency cells labeled by consecutive symbols (such as 6 and 7) are geographical neighbors.
3.4 Assigning DIGIPIN to coordinates coinciding with DIGIPIN Grid Lines
In some of the cases, coordinates of a location may coincide with DIGIPIN grid lines. In such scenario, the location is assigned a DIGIPIN symbol of the adjoining right-side (eastward) grid, if it falls on a vertical (north-south) grid line. Similarly, a DIGIPIN symbol of adjoining up-side (northward) grid is assigned, if location coincides with a horizontal (east-west) grid line. The coordinates coinciding with DIGIPIN line intersection are assigned to adjoining top-right grid.
The exception is made to coordinates coinciding with top-most (38.5° N) and right-most (99.5° E) grid lines wherein DIGIPIN symbols of the adjoining bottom-side (southward) grid and left-side (westward) grids are assigned, respectively.
3.5 Grid sizes at various levels
As explained above, the DIGIPIN generation is an iterative procedure. At level-1, the bounding box is divided into 16 (4x4) regions. The total latitudinal and longitudinal width of bounding box is 36°, resulting in 9° x 9° regions at level-1. Similarly, at level-2, each 9° x 9° region is further divided into 16 (4x4) sub-regions, resulting in 2.25° x 2.25° regions. The same process is carried out up to 10 levels.
The table below shows a relation between the DIGIPIN code length, corresponding grid size and approximate linear distance at the equator. It gives an estimate of the positional uncertainty associated with DIGIPIN code length. For instance, a DIGIPIN code of length 6 represents a ~ 1km x 1km region. It requires an additional 4 digits to precisely locate a 3.8m x 3.8m region inside this region.
| Code Length |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
| Grid Width |
9° |
2.25° |
33.75′ |
8.44′ |
2.11′ |
0.53′ |
0.13′ |
0.03′ |
0.5″ |
0.12″ |
| Approx. Distance |
1000 km |
250 km |
62.5 km |
15.6 km |
3.9 km |
1 km |
250 m |
60 m |
15 m |
3.8 m |
Table 1: DIGIPIN grid size at different levels
4. Key differences between Beta version and Final Version of DIGIPIN
- A modification in the extent of the DIGIPIN bounding box from 63.5° - 99°E & 1.5°- 39°N to 63.5°- 99.5°E & 2.5°- 38.5°N)
- The replacement of characters G, W, and X with C, F and T to maintain the phonetic and visual clarity of DIGIPIN, while maintaining the sequence of characters.
- A uniform gridding and labelling scheme across all iterations (1-10).
5. Illustration of DIGIPIN Generation Process
The figure 4 illustrates the complete procedure for generating DIGIPIN for a specific location. For example, the geographical coordinates of Dak Bhawan (28.622788°N, 77.213033°E) are marked with a red star on the India base map. The figure demonstrates the selection of DIGIPIN symbols at each level, based on the grid encompassing Dak Bhawan. The DIGIPIN of Dak Bhawan is 39J 49L L8T4.
6. Programming codes
6.1 DIGIPIN Programming Code
Function which takes latitude-longitude as input and encodes it into a 10-digit alphanumeric code is at Annexure 1.
6.2 DIGIPIN Reverse Programming Code
Function which takes a 10-digit alphanumeric code as input and encodes it into degree-decimal coordinates is at Annexure 2.
Annexure 1
I. DIGIPIN Programming Code
/**
* Function GET_DIGIPIN() takes latitude-longitude as input and encodes
* it into a 10 digit alphanumeric code
* A higher precision (upto 5 decimal places) of latitude-longitude input
* values results in more precise DIGIPIN
*/
function Get_DIGIPIN(lat, lon) {
//DIGIPIN Labelling Grid
var L = [
['F', 'C', '9', '8'],
['J', '3', '2', '7'],
['K', '4', '5', '6'],
['L', 'M', 'P', 'T']
];
var vDIGIPIN = '';
//variable for identification of row and column of the cells
var row = 0;
var column = 0;
// Bounding Box Extent
var MinLat = 2.5;
var MaxLat = 38.50;
var MinLon = 63.50;
var MaxLon = 99.50;
var LatDivBy = 4;
var LonDivBy = 4;
var LatDivDeg = 0;
var LonDivDeg = 0;
if (lat < MinLat || lat > MaxLat) {
alert('Latitude Out of range');
return '';
}
if (lon < MinLon || lon > MaxLon) {
alert('Longitude Out of range');
return '';
}
for (let Lvl = 1; Lvl <= 10; Lvl++) {
LatDivDeg = (MaxLat - MinLat) / LatDivBy;
LonDivDeg = (MaxLon - MinLon) / LonDivBy;
var NextLvlMaxLat = MaxLat;
var NextLvlMinLat = MaxLat - LatDivDeg;
for (x = 0; x < LatDivBy; x++) {
if (lat >= NextLvlMinLat && lat < NextLvlMaxLat) {
row = x;
break;
} else {
NextLvlMaxLat = NextLvlMinLat;
NextLvlMinLat = NextLvlMaxLat - LatDivDeg;
}
}
var NextLvlMinLon = MinLon;
var NextLvlMaxLon = MinLon + LonDivDeg;
for (x = 0; x < LonDivBy; x++) {
if (lon >= NextLvlMinLon && lon < NextLvlMaxLon) {
column = x;
break;
} else if ((NextLvlMinLon + LonDivDeg) < MaxLon) {
NextLvlMinLon = NextLvlMaxLon;
NextLvlMaxLon = NextLvlMinLon + LonDivDeg;
} else {
column = x;
}
}
if (Lvl == 1) {
if (L[row][column] == "0") {
vDIGIPIN = "Out of Bound";
break;
}
}
vDIGIPIN = vDIGIPIN + L[row][column];
if (Lvl == 3 || Lvl == 6) {
vDIGIPIN = vDIGIPIN + "-"
}
// Set Max boundary for nex level
MinLat = NextLvlMinLat;
MaxLat = NextLvlMaxLat;
MinLon = NextLvlMinLon;
MaxLon = NextLvlMaxLon;
}
return vDIGIPIN;
}
Annexure 2
I. DIGIPIN Reverse Programming Code
/**
* Function Get_LatLng_By_Digipin() takes a 10 digit alphanumeric code
* as input and encodes it into degree-decimal coordinates
*/
function Get_LatLng_By_Digipin(vDigiPin) {
vDigiPin = vDigiPin.replaceAll('-', '');
if (vDigiPin.length != 10) {
return "Invalid DIGIPIN";
}
//DIGIPIN Labelling Grid
var L = [
['F', 'C', '9', '8'],
['J', '3', '2', '7'],
['K', '4', '5', '6'],
['L', 'M', 'P', 'T']
];
// Bounding Box Extent
var MinLat = 2.50;
var MaxLat = 38.50;
var MinLng = 63.50;
var MaxLng = 99.50;
var LatDivBy = 4;
var LngDivBy = 4;
var LatDivVal = 0;
var LngDivVal = 0;
var ri, ci, f;
var Lat1, Lat2, Lng1, Lng2;
for (let Lvl = 0; Lvl < 10; Lvl++) {
ri = -1;
ci = -1;
const digipinChar = vDigiPin.charAt(Lvl);
LatDivVal = (MaxLat - MinLat) / LatDivBy;
LngDivVal = (MaxLng - MinLng) / LngDivBy;
f = 0;
for (let r = 0; r < LatDivBy; r++) {
for (let c = 0; c < LngDivBy; c++) {
if (L[r][c] == digipinChar) {
ri = r;
ci = c;
f = 1;
break;
}
}
}
if (f == 0) {
return 'Invalid DIGIPIN';
}
Lat1 = MaxLat - (LatDivVal * (ri + 1));
Lat2 = MaxLat - (LatDivVal * (ri));
Lng1 = MinLng + (LngDivVal * (ci));
Lng2 = MinLng + (LngDivVal * (ci + 1));
MinLat = Lat1;
MaxLat = Lat2;
MinLng = Lng1;
MaxLng = Lng2;
}
cLat = (Lat2 + Lat1) / 2;
cLng = (Lng2 + Lng1) / 2;
return cLat + ', ' + cLng;
}
