Raster and Grid Formats:
These grids are the most simple, because they consists of only a sequence of binary numbers, usually reading from west-to-east (in the geographic sense), and either from north-to-south or from south-to-north (as specified in the format description). Raw binary grids can also be encountered imbedded within more complex structures, such as those described below and in Self-Describing Formats.
Prior to the widespread use of Self-Describing Formats, which contain formally proscribed internal descriptive tags, many important datasets were published containing informal internal ASCII descriptions at the beginning of the file. Users are supposed to use ASCII editors to view the file beginning in order to find necessary usage information (e.g. number of rows, number of columns, etc.). One very important datum was the exact number of ASCII bytes in the file to skip over before reading the binary grid values. Here is a typical example (sample.ssmi.dat) from an old WOCE windstress dataset wherein the binary values appear as 'random' ASCII characters:
DATA SET : SSM/I WIND COMPONENTS, SPEED AND STRESS ; TIME AVERAGE : FIVE DAY ; TIME MIN MAX : 19900101 19900105 0001 0005 ; ARRAY SIZES : 720 360 ; LONGITUDE MIN MAX DELTA : 0.25 359.75 0.5 ; LATITUDE MIN MAX DELTA : -89.75 89.75 0.5 ; FORMAT EAST-COMP ARRAY : 720x360 1-BYTE UNSIGNED ; FORMAT NORTH-COMP ARRAY : 720x360 1-BYTE UNSIGNED ; FORMAT WIND SPEED ARRAY : 720x360 1-BYTE UNSIGNED ; FORMAT EAST-STRESS ARRAY : 720x360 2-BYTE SIGNED INTEGER (SGI) ; FORMAT NORTH-STRESS ARRAY : 720x360 2-BYTE SIGNED INTEGER (SGI) ; FORMAT COUNTER ARRAY : 720x360 1-BYTE UNSIGNED ; SCALING FOR COMP ARRAYS : WIND COMPONENT (M/S) = (X/4)-30 ; SCALING FOR WIND SPEED : WIND SPEED (M/S) = X/8 ; SCALING FOR STRESS ARRAYS : WIND PSEUDOSTRESS (M/S)^2 = ABS(X)*X/10000 ; VERSION : WOCE CD-ROM NASA JPL PODAAC V1-ATLAS2 199804 ; END OF HEADER : 1360 byte header ; s;onv;snv;r68of;9j;of;;;vn;sh.l;0o583473753700pppyup3hp3hjp38gvpvpghjgvpp8jgphgh ogppe0707aout;70an09'at[0ut[3j-nu-t3tnu[3[93j[g03jug[03j[mt[3u[0u[t3mu[3mu[3u[tu pj3nt037696e6[86;jg;e;ee;9898tutuut;n988-85--v'v-'n'tu'au'tu'tnu'9nu9nut9ut2ut2x ;otu3nu[386nau93u'93u'9u'-3'-93'-9u3'9u3'nu'39u'3ub93u'9u39'u3[u'3tu'3u'9'93u'tu [rest of file omitted for brevity]
These grids are some of the most widely used formats in the earth sciences, due to the enormous popularity of the Surfer gridding and contouring program. There are two basic types:
This example is a sea surface temperature grid offshore Namibia. Examining the small header we find these values: After DSAA (a company identifier), the number of rows, number of colums, minimum longitude (X), maximum longitude (X), minimum latitude (Y), maximum latitude (Y), minimum grid value and maximum grid value. 'NOTE: Blank rows between the data grid rows have been removed for clarity in this wiki.
DSAA 10 14 7 17 -29 -15 11.243040968085 19.431364974874 16.824741012143 16.745461989843 16.587686781674 16.253122497206 15.949882499559 15.870442610423 14.956398046048 14.164092764509 13.348231715184 1.70141E+038 16.864519096148 16.717397259789 16.573352923603 16.365521649285 15.861864168512 16.459582282843 15.828388339189 15.60694014831 1.70141E+038 1.70141E+038 16.832513910695 16.499913594476 16.681316331512 16.355517896677 15.844839501661 15.839318520163 16.100996884231 12.483563432934 1.70141E+038 1.70141E+038 16.912755112345 16.763857427221 16.365525179991 15.566948873345 16.140160100238 15.720448187103 14.727189252088 11.243040968085 1.70141E+038 1.70141E+038 17.143251105689 16.927550070385 15.490304247905 16.874353192752 16.153333513854 15.582507876994 14.260412739458 1.70141E+038 1.70141E+038 1.70141E+038 16.969848863574 16.764922793326 17.378931675059 16.620479695448 16.041638762935 14.784318533978 13.878714698118 1.70141E+038 1.70141E+038 1.70141E+038 17.111157870444 17.249487339579 17.076440987238 16.919177533376 15.815051098758 14.544624256724 13.902985069789 1.70141E+038 1.70141E+038 1.70141E+038 17.320508806953 17.010005719576 17.098764770367 16.320446004906 14.907059718288 16.016383730942 14.418986018669 1.70141E+038 1.70141E+038 1.70141E+038 17.417493510903 17.317982986081 17.360892518646 15.582766505059 14.907139087959 14.430006480511 1.70141E+038 1.70141E+038 1.70141E+038 1.70141E+038 17.430786856401 17.325174703284 16.320565414238 15.641073581356 15.132948057917 13.75622553285 1.70141E+038 1.70141E+038 1.70141E+038 1.70141E+038 17.509270321069 17.013905429559 16.159659152898 14.968126979467 15.354255678655 1.70141E+038 1.70141E+038 1.70141E+038 1.70141E+038 1.70141E+038 17.701093237905 17.354570863884 17.312330513577 15.541605807285 14.679367258808 1.70141E+038 1.70141E+038 1.70141E+038 1.70141E+038 1.70141E+038 19.241305214076 18.092511307966 17.146115195515 16.619967330077 18.668326557116 1.70141E+038 1.70141E+038 1.70141E+038 1.70141E+038 1.70141E+038 19.431364974874 18.797263012583 17.485058447247 17.114769871276 17.388525398727 1.70141E+038 1.70141E+038 1.70141E+038 1.70141E+038 1.70141E+038
Contains the same information, but all in binary and cannot be viewed.
Also known as the Arc Grid format and several other names. There are two versions of this format, ASCII and binary. They both have exactly the same explanatory header: for the ASCII version, the header is contained within the file, at the beginning; for the binary version, it is contained in a separate HDR file.
ncols 25 nrows 17 xllcorner -98 yllcorner 15 cellsize 1 nodata_value -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 23.73070 24.34900 24.77740 24.91330 24.82520 24.58710 24.27690 23.97770 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 -9.9999998E+33 (remainder of the file omitted for brevity)
Note that ESRI ASCII grids are "anchored" at the southwest corner by the X and Y values (of either the corner or center of that pixel, as appropriate), but the data values in the grid actually begin in the northwest corner.
ncols 25 nrows 17 xllcorner -98 yllcorner 15 cellsize 1 nodata_value -9.9999998E+33
The ESRI coverage format (see documentation link below) is an older format that can accomodate both vectors and rasters. Apparently it can accomodate about 15 different types of mapping concepts, corresponding exactly to the E00 compression format, also developed by ESRI. Coverages are often associated with the older ArcInfo software from ESRI, and there are indications that both are slowly losing importance in the GIS field. Coverages can also contain numerous separate files, and are often found compressed in the E00 format.
This type of grid is probably the most familiar of the grid-holding components of ESRI coverages, although there are apparently others. It is described in the ESRI reference below.
This topic is well described in the Wikipedia reference below. As found here without georeferencing, none of these simple images can be used directly in GIS systems. The major formats of interest to the marine science data manager are these:
With georeferencing, all of the image formats above can be used in GIS systems and be mapped correctly onto the earth's surface. If the image is projected, then complex methods in full GIS systems are required. If the image is not projected (i.e. it is in plain Cartesian coordinates) then it can easily be georeferenced by either of the following two methods:
In addition to the formats described here, raster and grid data can be entirely contained within more complex formats of the Self-Describing Formats type. In such cases the data values of the cells or the color values of the pixels are moved into an entirely different structure, often also containing metadata. The most notable of these formats are NetCDF, HDF and GRIB. NetCDF is unique in that it has an ASCII analog format called CDL, described above.
Gridded data can be placed into a special type of shapefile, called a "points shape," wherein each grid point is represented by an individual point on the map, with individual properties. The most obvious property for the point, of course, would be the original measured parameter value from the grid (i.e. the "z" value). When a large number of points shape points are viewed in a GIS system, they often appear to cover the entire map exactly like a colored raster image. But when the map is zoomed, each point appears separately. The value of points shapes is that they offer a very easy method to transfer data from complex grid systems (e.g. GRIB format for meteorological data) to GIS systems.
The XYZ spreadsheet/table format, where three columns of data represent (usually) longitude, latitude and a parameter value, is very closely related to geo-referenced rasters and grids. They are the simplest and most unambiguous means of transferring the contents of grids between programs. The relation between an XYZ file and the source raster/grid is not simple, however, as you can see in the related article Grids/Rasters and XYZ Files.